Sorry about the blurry pictures, I did this at night.
P.S. this is my first instructable, so bear with me.
P.P.S. This modification was based on the one available at: http://shop.lomography.com/holga-backup/35.html
Step 1: What you'll need
A Holga camera
Long, thin rubber bands (2x)
Cardboard (or any kind of padding material)
Step 2: Fix the Reel
Wrap the rubber bands around each edge of the take-up spool so that the hole is roughly the width of 35mm film. You can adjust the size by fitting the exposed end of your film into the hole.
Now place your spool in your camera. Use cardboard (or padding) to get it to fit in snugly.
Step 3: Insert Film Here
Push some through the spool's hole and twist the knob a few times to get it to take.
You'll need a lot of cardboard (or padding) to get the film canister to stay where it's supposed to. Put a bunch above and below it to get it to stay in place.
Step 4: Blackout!
Wait... what's this? Your viewing window for medium-format film is going to expose your 35mm film!
Fix this problem with a little electrical tape. Cover up the window and any other visible cracks that might let too much light in (except the shutter, we'll need that later).