Recently some friends and I have installed the Prusa Multi-Media Unit (MMU2) on our printers. I was struggling with the filament 'buffer' that came with it. It was difficult to load and only marginally effective at capturing the filament when unloading it.
I tried several different Spool Holder designs and settled on the ‘Multiple Mechanism Auto-Rewind Spool Holder’ (https://www.thingiverse.com/thing:3691892). Using it I was able to eliminate the Prusa filament buffer. My only problem was it was painful loading the filament between the spool and the MMU. I used a 1/2 inch by 1 inch long bolt and two nuts for the rack weight, but a friend tried a 1/2 inch by 2 inch long bolt with 3 nuts, which he felt worked better.
My solution to make loading the filament easier, adds a manual extruder mechanism that I can lock and crank, to push or pull the filament between the spool holder and the MMU.
I haven't tried it, but you should be able to glue the winder and the extruder to an existing spool holder.
It uses a 5mm steel ball and a spring that I hand wound from piano wire as a detent for the Base Winder Cam to hold the Base Winder Lock Lever in position.
A PC4-M6 Fitting can be screwed into the Base Winder Main to hold the PTFE tube to the MMU.
Here are some of the parts that I used:
WINSINN MK8 Drive Gear Extruder Hotend Feeder 40 Teeth 5mm Bore Stainless Steel for CR-10 CR10 Prusa I3
HUABAN 10pcs 3D Printer MK8 U604ZZ 604UU 4x13x4mm U-Shaped Bearing Extruder Round Wheel Gear Walking Guide Rail Bearings Fitting Pulley
Eowpower 10Pcs 100mm x 5mm Ground Shaft Round Rod for RC Helicopter Airplane Car
All the screws are M2x10 flat head, except for those noted on the figures.
I provided Rack Side B a and b STLs as an alternative to printing Rack Side B laid down as one piece. Print Rack Side B a STL with supports, and Rack Side B b without, then glue them together with Acrylic Cement.