At Joel and MyMinifactory's request, I printed my first version, found here: https://www.myminifactory.com/object/3d-print-71297.  It worked well enough, but the perfectionist in me said that I could make it better.  Well, this is it.

First, I found that the grips were a bit tight for my .75" width wood shelf.  I had made them 18mm(.71"), so I decided to widen them slightly to .74".

I noticed that one of my smaller, 1lb spools would make the filament guide touch the opposite spool bracket, so I shortened the length of the filament guide some, just in case Joel had thinner spools to use.

I had also noticed that if I had a spool with more filament on it, the filament could catch and touch the inner spool holder brackets of my "whole" version, so I curved the inner spool holder brackets to avoid this issue.

Like my previous version, this one requires 4 608zz bearings 8x22x7mm (you can buy them cheap here, but long delivery time: https://www.aliexpress.com/item/20Pcs-608-2RS-608RS-608rs-608-rs-Deep-Groove-Ball-Bearings-8-x-22-x-7mm/1716190815.html?spm=a2g0s.9042311.0.0.229c4c4dGIFYy0).  Otherwise, any hardware store has them (for a price!).

The fit between the bearings and the spool brackets should be nice and tight.  If they're not, you may need to tweak your extruder/x-y comp settings.  Even if they are a little loose, don't worry- mounting them on the wood shelves will keep them together.

Here are the Pros, as I had listed in my previous version:

Pros:

1.  Ball Bearings! (Less Stress)

2.  Clears your brackets! Place the spool holders anywhere along the shelf, your shelf brackets won't get in the way!

3.  Easy and fast to print!  No supports necessary.

4.  Place them side by side!  You can have multiple spool holders that won't obstruct each other, so you can leave them under your favorite filaments, even if they're right next to each other.

5.  Filament Guide!  Funnel-like guide helps to prevent filaments that are on the far left or right of your shelves from tipping over.  I've designed it so you don't have to disconnect it from the printer/extruder to thread the filament.

6. Weight distribution designed to minimalize stress on the spool holder, avoiding potential cracking and breaking.

7.  2 Versions available, each with their own advantages.  The 2 can also be mixed together.