The standard spool holder that comes with the Prusa i3 MK3 does not cater for spools with narrow or small holes in the middle. You can replace one or both of the holder spigots with this design to use spools with small holes like the Sainsmart TPU filament.
Spools with holes as small as 28 can be used with it.
Note. Print two of the lugs and set the infill to about 70% for just the lugs as a fair amount of strength is required to connect the new spigot and twist into place. Ensure the lags fully inserted into main piece.
I used PLA and it seems fine but ABS might be a better option.