I started this project from tdanro's "Adjustable spool holder for the Original Prusa i3 MK2" (https://www.thingiverse.com/thing:2122419), but it evolved into a shelf mounted unit, and the capacity was enlarged somewhat.

This is admittedly total overkill, but I had some of the hardware already & just wanted to learn more about putting together a more complex project in CAD.

It's designed to print with default settings and without supports (at least it prints fine without them on my Prusa MK3, but YMMV).

The printed parts/quantities:

(1) Center.stl (1) Center_top.stl (1) Pinion_bottom.stl (1) Pinion_top.stl (1) Knob.stl (2) Slide.stl (2) Rack.stl (4) Roller.stl (4) Standoff.stl

Hardware:

(2) 8mm x 200mm shafts https://www.mcmaster.com/6112k44
(4) M8 x 1.25mm socket head bolts https://www.mcmaster.com/90128a276
(4) M8 x 1.25mm nylock nuts https://www.mcmaster.com/94205a260 (4) 8mm LM8UU linear bearings https://www.mcmaster.com/61205k75 (4) 608/skateboard Bearings https://www.mcmaster.com/5972k91 (4) 4-40 socket head screws* https://www.mcmaster.com/91251a149

• The socket head screws I used were 1.5" long to mount on a half inch thick shelf. any similar sized machine or wood screw would work, though you might want to change the mounting holes to accomodate.

Assembly:

Pretty obvious I think, except for the pinion gear. I split that in two to make the print simpler. It aligns and joins with two pieces of scrap 1.75mm filament.
(BTW - the best glue I've found so far for PLA is DAP Rapid Fuse). Once glue is set use an x-acto to clean up any excess glue and debur.

The LM8UU bearings seem to vary in size somewhat. I've sized this for typical metal cased bearings, but also had some drylin bearings that were fairly loose.

Note that the standoffs go between the bearing and the slide so that the bolt can be tightened while still ensuring that the bearing turns freely.