Shelf-mount hanging spool holder

A filament spool hanger designed to mount on the edge of a shelf - it can be used just to store your filament, or if the shelf is close by you can feed your printer directly from the spools where they hang. I wanted to keep it simple, flexible, and 100% printable - no metal parts are required (bearings, rods, etc.). But if you want to combine some metal parts into your implementation these hangers will work with that too. The STLs included here are sized to fit the shelf above my printer, but t