One thing I did yesterday was tape over 2/3 of the paper slot on my 126 backing paper. Then I taped 35mm Plus-X film to the paper and rerolled it into the cartridge. My theory was that only 1 or 2 sprocket holes would be open to catch the pin.
I put this cartridge in an old Instamatic 104 (very simple-minded camera) and so far it seems to work. I will use it this week and see what happens.
On another track....
I have a thought about a fun way to make 126 film. Using a bulk spool of unperforated 35mm film, create a roller device with a measured punch that will place a sprocket hole at the appropriate position. Roll the film through, punching holes and then load segments into 126 cartridges or modified 35mm spools.
Probably way more hassle than it's worth though, given the toy camera nature of Instamatics.