Free spinning spool holder that mounts magnetically. I printed the 3" version. In retrospect, it could have been longer. If it gives me any trouble I will print out the 4" long version. Designed for Ultimaker brand filament spools. The cart we put our Ultimaker2 on has steel side panels.1x 5/16"-18 x 3" Full Thread Hex Bolt 2x 5/16"-18 Nylock Nuts 1x 1/4 flat washer 4x 608 bearings (probably don't really need this many) Disc Magnet Mount I found at Ace. I had to open up the hole in the magnet from 1/4" to 5/16" Also, the holes for the bearings are a bit tight. I had to squeeze them in using a bolt and nut (and a little hammer). I put a very thin rubber band around the spool holder to grip the spool and make sure it doesn't walk off the holder.