Round urethane belts?

https://www.vxb.com/5mm-Round-Urethane-p/RBO-5mm-Round-Urethane-3-16.htm

https://www.amazon.com/uxcell-Transmission-High-Performance-Urethane-Conveyor/dp/B07SD5XNJQ

Get used in food conveyors a lot.

They are a bit stretchy though. Don't know if that works for your application. Can't really remember tensions on these things, but I thing you probably want more of a 10mm belt to be hauling around 20 lbs.