*I made a 2nd, improved version here: https://www.myminifactory.com/object/3d-print-71945.
Hope you have 4 608 Bearings, Joel! I've always preferred having some kind of bearing system for my spool holders. It means less friction, which means less stress on your extruders!
I have 2 designs here; one is divided and simplified, hanging off each of your wood boards, the other has brackets that reach across your wood boards. Not having the boards myself, I couldn't test my first design, so I'm including the second as a fallback.
1. Ball Bearings! (Less Stress, as mentioned above)
2. Clears your brackets! You don't have to worry about your brackets being in the way when placing the spool holder.
3. Easy and fast to print! No supports necessary.
4. 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! 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 thread the filament.
6. Weight distribution designed to minimalize stress on the spool holder, avoiding potential cracking and breaking.