This roller is designed to fit 1/2" EMT electrical conduit. To make 1 roller, print 2 pieces. Each half of the roller accepts a 608 bearing. I printed this, in PLA, with the bearing hole towards the print bed. There is an integral support in the middle that can easily be removed after printing.

I also found the tolerances to be a little tight on the O.D. of the bearing. A standard 608 has an O.D. of 22mm. I made the hole 22.2mm, but it could be made a little bigger. The problem is that you do not want any play after the bearing is installed. That would effect the accuracy of the part ( If you need accuracy).