I like the Polymaker Polybox (keeps filament dry while printing) however it annoyed me that they only fit two spools, even though Polymaker themselves offer thinner and smaller spools. This mod allows a 3rd, smaller spool to fit inside the Polybox while still being mounted on a bearing for smooth spooling when printing. This way you can (for example) print in two colours and have dedicated support material!

Many support filament spools are smaller as are the Nylon spools and other expensive/speciality filaments. Fits spools with holders with inner diameters of 50-60mm and overall diameter of 165mm (limited by Polybox width).

You will need:

3x M3 nuts 

3x M3 screws (20mm) 

Assorted washers (to fit M3 and M6)

1x M6 Screw (30mm approx)

1x M6 Nut

1x 608ZZ bearing (this is a really popular bearing in 3D printers). 

Drill for making holes and smoothing bores if needed.

This mod is a top plate, some spacers and a "screw in bottom plate".

Place the M6 screw head side down in the "top mounted plate" with the supplied plastic M6 spacers (these keep it centered). Note these are different, the thicker one is for the top side.

Mount the "bearing clamp" to the "top mounted plate" with 3x M3 screws. Ensure the M6 screw from the previous step still spins easily.

Place parts on one side of filament spool. Insert and thread the "threaded holder" into the other side (not too tight, remember these are plastic threads!).

Note: The pitch is large for the screw in piece, print these parts at around 0.15mm layer height and ensure threads are clean and clear. Drop some oil on them to smooth out operation. 

Drill (M6 sized) hole in top of Polybox (there is already a dimple from injection molding right in the centre). Place some metal washers on either side of the hole in the Polybox cover and mount complete assembly into Polybox. Holding M6 thread with something and secure with M6 nut. 

When replacing filament spool you just need to unscrew the bottom holder.

I drilled a new 7.5mm hole in the clear plastic of the Polybox cover and re-routed a rubber grommet and the filament to be inline with the new spool position.