In the various incarnations of our arms and handlers we drove the flat belts in a couple different ways.
1) using rollers made from 1" OD aluminum tube. Where the belts were located, we wrapped the rollers with 1/4" wide strips of 1/16" thick bump-on adhesive backed urethane from McMaster.
2) using 1" wide crowned rollers that were 3D printed. STEP files for these parts can be found on the CAD models found in
this thread. We made rollers for both 1/2" hex shaft and 3/8" round keyed shaft.