This is a mix of many great ideas I've found here. Not fully tested, but works so far.
Customizer link for hub adapter:
Update: Hinge arm clip is now a little thicker. It snaps, but feels too soft.
Changed 4mm holes in left arm to 4.2 so M4 screws don't thread through 2 things.
Gantry spool arm by sdi811
Side extruder mount by tierradenadie
(I had to enable supports for my additions)
The original is by Alpharic along with the extruder spacer
The parametric hub is remixed from AndrewBCN, and a few others
50% infill was used for everything.
The hinge arm uses M6-100 and nut (M6-90 should be fine)
Hubs use 4x 608ZZ bearings, and 2x M8-45 and nut (M8-35 would work on the right side), and a washer or shim on the left.
2x M4-15 can be used to secure the left arm (4mm holes so they thread in). Not really sure it's necessary though. It was just done to space it to 75-80mm wide spools.
The inner hubs are 0.3mm smaller than the spool adapters to slide in and should be about 10mm smaller than the smallest spool opening. Then the adapters should be a quick print, and have some space at the flex point between the inner ring and outer tabs.