I have my 3D Printer in IKEA closet, and I used to put the spools through the clothes rack, but since the rack have a weird shape (rectangle) and the spools are rounded, they didn't move freely, so I looked for a way to have a bearing that can be inserted in the rack and at the same time the spool and have free rolling. so i made this after many many trials and errors.

the exact size might not be fitting to all spool sizes, so (after trying with multiple spool sizes) I made a handful of designs with different sizes (the difference is in the rounded part that gets inside the spool).