I have slatwall behind my 3D printers, so use that to hang everything from. Filament, tools, etc. For simplicity's sake, I took a standard 8" hook and cut off the curved tip. Then I attached a 12" ¼-20 threaded rod using two small hose clamps. I covered the clamps and hook in heavy-duty shrink-wrap - not necessary, but looks nicer. This resulted in a threaded rod sticking out perpendicular to the wall. I wanted a slight upwards angle, so I attached some foam padding to the slatwall base on the hook.

Then I used a pair of ¼-20 nuts to adjust the depth where the spool would touch. Two nuts so they would lock in place. Same on the other side.

The spool has three parts - makes printing easy and no supports needed. Standard bearings press in to either end of the shaft. I prefer “fidget spinner bearings” because they spin much more freely than skate bearings. Standard 8x22x7mm.

The shaft has small lip - the large disc drops onto this. Depending on your printer tolerance it's a snug fit or may have to be glued. The end cap threads in place. It's a simple design but it works with any spool. I've been using it for over a year and have improved and simplified the design a few times.

The spool should have room on the holder so it can spin freely. The holder is close to the printer to prevent the spool from unwinding on it's own.

Note: My previous version had a left-hand thread. This one has a right-hand thread. I was playing around with ideas.