I recently built the MK3-MMU2s enclosure posted by Strita, it can be found here https://www.prusaprinters.org/prints/3673-prusa-printer-enclosure-v2-with-mmu2s-support. The stock MMU2S filament buffer wasn't working too well, the filament would feed all the way back to the spools when it unloaded and would come off of the spools, then just hang up the next time the filament was loaded. And the stock PTFE tubes really don't line up well with the spools. So I designed this little bracket to hold a pass through PTFE coupling, and then screw into the top of the enclosure. You may have to experiment with the placement of the brackets, but I was able to get mine to line up better with the spools and apply just enough friction so that the loose filament stays in the buffer, and doesn't retract all the way out to the spools. I had to tighten the extruder hatch door screw a little to accommodate the extra friction on the filament, but not much. When I first tested this setup I noticed a little bit of under extrusion, tightening the door screw about one turn was enough to fix the issue.

I used 6 x 1" sheet metal screws I had on hand to secure the bracket to the top of the enclosure. The diameter of the screw holes is set to 4mm, so any self-tapping screw of that size should work.

The pass-through couplings that I used I got off of Amazon: https://www.amazon.com/gp/product/B01IB81IHG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Print instructions

It's a pretty straight forward print, no supports needed, and I used 20% infill, with 2 perimeters.

I printed mine out of PLA.