We're using the yellow 1/4 inch polyurethane shingle routhane tubing from McMaster. Make sure you buy the barbs that hold them together. Don't pay for the $300 welding kit or try to weld them yourself because you'll have belt failures and it's EXPENSIVE.
We have them cut about 10% shorter per foot to make up for stretch. We also have spans of no more than 1.5 feet between rollers.
We've had 0 belt breaks, so they work
.