For my delta, the top pulley are supposed to be comprised of a flanged bearing. However, I had a bunch of 9mm diameter x 4mm wide bearings with m3 holes in the middle, which fit the bill with the exception of missing flanges. I created a flanged spool that lets you slide in the bearings, but have come up with something simpler. These simple discs with M3 holes in the middle. I printed out 6 of them at 100% infill. They are 1cm in radius, 1mm thick, and have an M3 hole in the center. Basically insert them in, add two of the 4mm wide bearings, and add on in at the end. Now you have flanges up to prevent the belt from slipping off of the bearings. I made sure the smooth side(side on the tape during printing) is facing toward the belt.