Keder cord

Try Larry’s redneck approach, Aka Rzrbug.  While he is no with us, many of his redneck ideas will live as long as this forum does. 

https://tab-rv.vanillacommunity.com/discussion/comment/5126#Comment_5126

I've used 5/16" plastic tubing available in most big box plumbing departments - there's a hard version thats opaque vs a clear soft version.  The hard version works better.  I've made a 9 x 9 tarp with it and it works well. I chose to make a sleave in the tarp and just push the tubing in when putting it up. The tarp folds much better that way.  Here's a link below - it costs all of $2.39 at the moment.

https://www.homedepot.com/p/Everbilt-5-16-in-O-D-x-3-16-in-I-D-x-10-ft-Pre-Cut-Polyethylene-Hose-HKP003-PE004/303132505