The latest of the many spool holders I've designed, this one is designed to hang off the front face of Joel's filament storage shelves for easy placement of spools anywhere they are needed.  Simply drop a couple of brackets onto one of the 1x2 boards wherever is convienient, slide a printed rod through the spool, and drop it into the hooks.  With convienient holes near the top of each bracket to tuck in the end of an unused filament if the need arises.  I avoided fillets and chamfers all around on this one and tried to match the same design asthetic as the shelf brackets already on the wall.  The spool-brackets all use the same 5mmx12.5mm walls as the shelf brackets already on the wall.

These brackets use the same standard 16mm-diameter printed rods that all of my spool holders use.  I like the 190mm long rods myself, but feel free to take the 100mm rod and scale it in Z only to whatever length works best for you.  100mm as an original length makes for easy math - want a 150mm long rod, scale it to 150% in Z and print.

Three different versions of the bracket are available, depending on your preference and the size of printer you have available.  The 'Full Bracket' is going to need a printer with a large bed, but is a an easy one-piece print and is then ready to use.  The two others both require a second extension piece to be long enough to reach the next shelf down to avoid putting a lot of twisting force on the shelf holding the weight.  The 'Bracket for Printed Extension' uses the 'Printed Extension Piece' which slides into the slot at the bottom.  A hole and nut-trap is provided for an M3 screw/nut, but anything that keeps the piece from falling out will do - a zip-tie through the hole, a drop of glue in the slot, whatever you have available is fine.  The 'Bracket for Wood Extension' works the same way, except the slot is sized to take a scrap of the same 1x2 lumber used on the shelves for its extra length.  Again, a hole and nut-trap for M3 hardware is provided, but anything that keeps the wood from falling out is fine.  The 'Wood Alignment Spacer' part is optional - to be attached to the bottom of the wooden extension to keep things at a perfect 90-degree angle hanging down, but you can skip it if you don't mind things hanging at a slight angle.  It's got a hole for a countersunk flathead screw (don't screw so tight as to crush the plastic), or it could be glued in place.

Pictured as an example is both a bracket with printed extension, done in Filaments.ca Econofil Black PLA on a Prusa MK2, and a Full Bracket done in iprint-3d.com New-White PLA on a JGAurora A5.  The exampe refill spool pictured is mounted on a snap-together printable spool also of my own design, available here: https://www.thingiverse.com/thing:2929753

Feel free to also check out the other options in my collection of spool-holder designs:  For mounting on a Prusa MK2 frame (or MK2S, MK3, probably clones, etc.): https://www.thingiverse.com/thing:2049635  For mounting to a 3030 aluminum extrusion: https://www.thingiverse.com/thing:2340601  Or a parametric fully customizable option to clamp to and hang from a shelf or other similar object: https://www.thingiverse.com/thing:2102971