The 320s come with a very nice aluminum tongue box with propane and battery box and room for much more, along with a nice looking platform begging for more storage boxes. I've been scouring for a year, seen many mods online to completely replace the existing box but had difficulty finding boxes I could add that were the size and look I wanted, most are too wide (I've seen 24" mounted on the 400s but the 320s are smaller).
This week I found these:
https://www.amazon.com/gp/product/B0058TACFQ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1. At 18" wide, 8" deep and 18" high seemed like they'd fit the bill, and indeed they do. I just finished mounting and thought I'd share.
Simple install with a drill and a little hardware from ACE and Home Depot. The sharp inner edge wasn't lined so I added some electrical tape to prevent scrapes from anyone reaching inside.
These boxes come with a lock which seemed great, but after loading one of the boxes and trying the lock, now it won't open. I'm hoping I can do this without a locksmith, we'll see how that works out. Other than that, I'm thrilled with how they look and the extra room we've got.