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.
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https://tab-rv.vanillacommunity.com/discussion/comment/5126#Comment_5126
2021 T@B 320S Boondock “The T@B”
Towed by a white 2019 Ford F-150 4x4 Supercab, 3.5L V6 Ecoboost “The Truck”
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