I know that you love giving high fives all the time so I thought that it would be fitting for you to be able to give the arm that supports your precious filaments a high five every time you decide to change your spool out.
I used an open source hand SVG from Wikimedia Commons -the free media repository (because I'm not too great at drawing hands) I hope that you don't mind? Here's the link to make sure that they are properly credited: https://commons.wikimedia.org/wiki/File:Hand_5.svg
When it comes to the actual design of my spool holder, it's pretty simple. I've designed it to be a counter lever that hooks over the front rail and under the rear rail of your shelving system. You should be able to slip it through the gap between your rails and the wound filament on your spools even if they're relatively full because spools are never filled right up to the edges even when new. This means that you can easily reposition the holder to any part of your shelving system even if it's being used (Unless it's got a large spool laying sideways on it.)
Printing: I'd just use your usual settings for whatever material you'd like to use. This shouldn't be a high-stress part so it's not terribly important - Just make sure that you have a decent wall thickness where the threads for the screws will go on the spool arm.
The arm is designed to be printed on a large flat face at the bottom of the model and the support/hook is designed to be printed on its side. You might need small supports for both parts.
Assembly: You'll need a screw -Just anything you have laying around. The longer the better. (and maybe superglue but it's not crucial)
It should be easy to figure out how these two pieces slot together. I'd put a bit of superglue in the joint just for added strength. Once you've slotted the pieces together then screw them together. There is a small hole in the hook behind the screw hole that you can put your screwdriver through which should make it easier... and that's it. you're done!
(maybe this could have been one part but I was worried about the length of this and wanted you to be able to print it on a smaller printer so you can use the larger printers for other things... you're always printing huge projects!)
Thanks for doing what you do Joel. See you next time!