SITE GLASS FOR 320 ALDE RESERVOIR

But @Verna, how else would I get such a great calisthenic workout? Stretch, bend, contort, remove two screws without loosing them in the abyss, contort again to look, reverse process. 

@ScottG, you up to blazing a trail by designing a workable sight (hole) for those poor, unfortunate '21 t@b owners? I know you practice t@b yoga, and you do mods, so you're most qualified in the particulars of teardrop bathroom yoga. Although your t@b is a different design, how about assisting fellow forum members develop a solution. Oh, my, do I hear power tools revving? Enjoy (teehee)!

@gulfarea, good idea, Art.
Here's what I am looking at doing..

Here's a mirror that should work, which has a hole at the top from which to hang it..
Coghlan's Featherweight Mirror https://www.amazon.com/dp/B000RZQI68
I plan to use a small LED light in the corner behind the tank & wire it to a small rocker switch that might fit on the plastic panel below the 12V sockets & wired to the same power lines.
A peep hole (one inch maybe?) could be drilled at the position shown, and covered in any number of ways to allow an easy view by simply holding a cell phone camera up to the hole.
The mirror (not the one in the link above) was digitally inserted into this photo from another photo taken from the same angle near the vertical edge of the wood, because I wanted to show this wider view.  It should work, as long as you locate the hole high enough & far enough to the left to see over the edge of the tank & avoid wires & tubes on the inside, which should be approximately the position shown.  You will also have some latitude for positioning the mirror left/right & up/down for optimal viewing.

Glad you found it useful, @gulfarea. 
I hope to get started on this soon, in case you need a guinea pig to see how it works out.  I'm just waiting on a couple of parts to be delivered today, then I'll add some followup.

I'm happy to report that I've completed this project & I'm happy with the result. I was able to add a new light for the Alde tank, a switch to control it, a mirror, and a peep hole to view the tank level.
I will list links at the bottom for the parts I used from Amazon.  For the light switch, I used a toggle switch left over from my past mods to add power-saving kill switches to the Alde & Jensen units.
The first thing I did was to disconnect the spade connectors from the 12-volt outlets above the glycol tank & carefully pull the wiring out as far as possible to make it easier to add a new wire pair for the new tank light & switch. I had previously replaced the USB sockets with a second cigarette lighter socket, by tapping into the existing source wires, so the thinner gauge wire pair from the old socket was still available to extend.  So I used that pair to solder on a new wire pair to feed the light/switch.  It is also possible to tap into the main source wires feeding the existing socket.
Below is how I connected the LED light strip & switch to the extended wire pair..

More of the wiring below, and the holes I drilled for the 1" peep hole & 3/4" for the light switch.  The envelope taped to the wall is to catch the sawdust - less to clean up this way.

Below the 1-inch hole saw drill bit used for the peep hole and the 3/4" spade bit for the switch..

The wiring is in place, and I added a zip tie to help keep it tidy..  In the corner behind the tank is a loop of 14 gauge household wiring that I attached to the top screw hole in the light strip which I stuffed down in the corner.  It was a snug fit, so I wanted to be able to pull it out.  The black strap is double-sided velcro, which I normally use to secure a small bottle of glycol stored in the corner..

Below, I have added the mirror, which also shows the glycol level from this angle..

And a view through the peephole..
(The only "issue" here is figuring out what "XAM" and "NIM" mean.  )

Everything back in place..
(I put a piece of thick foam under the spare bottle of glycol to prevent it from bouncing around, and I suppose it could absorb any drips too.  It has survived a trip from VA to ME, so it works.)

And finally, the crowning jewel - a cap for the peep hole made of silicone rubber, which is a perfect fit for the 1-inch hole.  It is soft and bendable, so easy to fit in or pull out, but otherwise very snug.  I tried to keep it located to the left & away from the corner to make it easy to get a direct view.  Or, I can use my cell phone camera to view or document the glycol level.


This project took me a bit longer because of the below incorrect information provided in the Amazon product information..



As a result, I had to disassemble and rewire a couple of connections, recrimp new connectors, replace the heat shrink tubing, and also replace a blown 15-amp fuse.
The black wire on these lights was the negative - I should have checked it myself, which I did after the fact by using a spare wired 12V socket plug with wire connectors on the end.  But here's an easy way to check..


Here are the parts links..

@BrianZ, thanks. Almost ingenious. I have the "xam" and "nim" thing handled, though. Severely dyslexic husband will get a correct reading each every time.

Nice job @BrianZ, I would have been disappointed with anything less...are you sure that bottle contains Alde fluid???  

I just got up from a nap and had an idea... This gets a little more involved but, if you had a clear tube that came off the bottom of the reservoir tank and then looped up where you could see it wouldn't the level of the fluid in the clear tube mimic the level in the reservoir?  With marks on the tube or where the tube mounts you could set your NIM & XAM level indicators and know the level at a glance.  In fact, I think you could loop that tubing back into the top of the reservoir and not worry about "squirtige".
Ok, I feel another nap coming on...

@Tabaz , nope, never thought of that, but then I think I've only added a little glycol maybe once in 4 years, not counting the recent full replacement after flushing the system.  I used a small flashlight in the past. I think your tank may be easier to get to, without the side cabinet in the way.

@Tabaz, yes that's an excellent idea.

In fact, you could run the tubing all the way to the dashboard, for continual monitoring, just in case!

Of course it would need a bleeder valve, in case an air-lock gets in there, and regular flushing, of course, to keep the tube from developing any corrosion.

PS:
@fstop32, You reminded me of the time years ago when I helped my dad build his backyard pond, by providing some younger muscle for digging it out & hefting the many rocks.  One issue was the slight slope of the backyard where we were digging, so we had to build it up a bit at the lower edge, like a dam.  The question became "How high should we make it?", because he wanted the rocks around the edges to be even with the surface.  He said, holding up a long piece of clear vinyl tubing, "That's why I got this!".  By filling it with water until both ends were level with the higher edge of the pond, we could then move one end next to the lower edge & use that level to guide us.

Ahhh @Tabaz, I need to learn to curb my smart remarks until after I've finished my coffee!

@fstop32, it is indeed a good idea! Could even be artistically mounted in black walnut, methinks!

And no lack of respect intended, for your nap-ideas! A majority of my most creative 'thinking' happens when I'm half asleep! (Hmmm, I probly shouldn't admit that)