A stiff piece of tape is all you need. Feed the tape in and roll the film backwards with it a bit. You have to kind of rewind until you feel it "pop" around the exit ramp, then put the tape in. I even had a specific tape dispenser with a metal tongue to help feed into the canister when I used to work a 1-hour photo machine at a Walgreens.

I've had to do it at home or on the fly more than once. Tape will work there as well, assuming you can feed it past the opening (maybe fold it over itself for a couple of milimeters?). It takes a little practice and finesse, but it's the safest way ... meaning you don't get nasty chemicals on your tongue!