Well you're further along than I am, I'm stilling cutting drive rollers... making 6 conveyers of various lengths simultaneously.
That size of tube looks about the size of tire on the back of my combine.
I used 15" car tires, rubber isn't too thick & the tube had mold? lines, so I got the cuts fairly straight with scissors. And the belts are 60 - 75mm wide.
I was just going to stretch the rubber belts a couple inches further than its actual length for a firm fit. No spring loaded belt tensioner. The drive & rear idler (as well as rollers thru the middle) will be running on flanged bearings I got from Boca Bearing (com). Also, I didn't cross cut the belt, there's no splice, it's an endless loop.