Flat belts worked really well for us on our collector:
https://www.youtube.com/watch?v=yCBjZ0s0kAo
We used McMaster flat urethane belt with this part number: 6075K14
To make sure the belts tracked well, we wrapped crowns of tape over the 1/2" shaft (which raised the diameter of the "pulley" to the minimum diam. specified for the belt product). We usually wrapped a few layers with the upper layers thinner in width to create a gradually crowned shape. In our experience, the belts have run great as long as the crowns on each shaft are properly aligned to each other.
I will edit with the exact name of the tape product we used if I can find it. It was in the electrical tape section, but it is not smooth on the outside as most electrical tape is. Instead, it was black and fairly sticky on both sides, which made it pretty good for our purposes. We acquired it from OSH, a local hardware store chain, but I cannot find the product on their website.
Hope this helps.