This will require access to a 3D printer as well as doing a slight modification to the camera to make room for the wind knob. If you decide to use the original Polaroid back, it will need permanent modification so it won't be able to use pack film anymore. Otherwise, you can 3D print a back to work with this conversion.

Printing up the parts

Download the 3D files here and print 3 spools and one of each of the other parts. The knob and drive spool should be printed with 100% infill for strength, and the back should be printed with supports everywhere. You will need to sand down the spools and receptors to 600 grit to reduce friction, as well as any place the film slides along. The film pack is designed at 89.5mm, but printers will often add .5 or so mm to the print job and it won't fit until it's 89.5mm wide. Same with the pressure plate. Be sure all parts fit OK without forcing them. The hinges and latching hooks on the back will need filing to fit properly, and the drive shaft hole in the side of the pack will also probably need filing so the shaft fits.

Preparing a Polaroid back to use 120 film in 6X6 format

A Polaroid back has lots of things in it to make it work with pack film, most of which needs to be removed to allow the 120 film spools to fit. It's easiest to work with a back without a development timer, so, start with a plain back and do the following:

Remove the back by pushing out the hinge pin. If you have a newer camera, plastic push pins will be inserted at each end to keep the hinge pin in. You'll need to remove one of them by prying it out. If it breaks, push it in with a punch/narrow screwdriver until the other one pops out. Pull it out and push the hinge pin out the other direction.
Drill out both rivets on which the rollers pivot open
Pry up the sheet metal that holds the film guide spring and remove the rollers.
Insert the 120 film pack and mark where the spool edges line up on the back and cut 2 grooves into the inside of the back.
You will need to cut through the aluminum to the hinge. The high point of the spool is at the left hole in the aluminum in the picture. Paint over the metal.
Mark the location of the wind shaft and file a groove into the side of the back with a rounded file.
Cut off the two roller springs to make room for the takeup spool.
Make cuts as shown and pry out the metal to make room for the peep hole.
Drill and file a hole for the peep hole. I lined the outside of the hole with craft foam to seal out light. The pressure plate fits precisely and pressure should not be placed on the peep hole tube.
The finished back should close easily over the 120 film pack and the wind knob should turn with minimal resistance. If there is excess resistance, be sure the grooves filed into the back are positioned correctly, and that the spool ends are sanded smooth. The foam seal between the peep hole and the peep hole shaft on the pressure plate should not press against the back or it will press the pressure plate down against the film, providing extra friction.

Preparing the camera to use the 120 6X6 back

Insert the 120 film cartridge and line up the drive hole with the edge of the camera, file a U shape where it lines up to give clearance for the drive knob
If using a 3D printed back, install the back with the hinge pin and cut a light seal out of craft foam for the peep hole. Glue it onto the back being sure it lines up on the outside of the peep hole to prevent any undue pressure on the peep hole tube.
If using a modified Polaroid back, place craft foam along the edges to prevent light leaks. The 3D printed back should seal out light adequately, and the seals may put undue pressure on the hinges and latch hooks.
Place some hockey or gaffer's tape over the gaps between the edges and the film retainer to prevent any light coming in along the edges.

Using the 120 film holder

Place the spool adapters onto the film rolls and the drive wheel on the bottom of the right empty spool
Thread the start of the 120 film onto the empty spool and place the two spools into their receptors.
Place the pressure plate over the film and make sure it is seated correctly
insert the knob and verify that the film is attached to the takeup spool
Insert the adapter into the camera. Be sure it is firmly bottomed out in the camera.
Close the door and advance the film to the first exposure. If there is too much friction, you are at risk of breaking the knob. Stop, unload the film in a changing bag or darkroom and investigate the source of friction.
The cartridge should have just enough friction to wind the film onto the takeup spool so there are no light leaks when removing the film from the camera.

Feel free to contact me with any questions or news about your build!