Prusa supplies SpoolHolder_B4.STL for a MK2/3 reel holder. However with a heavy full reel it can droop because it is one-sided. This piece, adapted from the Prusa file to insure compatibility, provides a second support to plug into the holder to stop any droop. The main holder still bears most of the weight, so this piece can be much smaller.

Print instructions

I used 20% fill, gyroid with PLA.