I bet taking the seat off your toilet would work if you really needed a seat. Squatting or aiming would work in most cases.
Good point .. in a pinch, for a pinch .. LOL
I found a Youtube video where the guy mounts the toilet seat with the hinge pieces up against the rim of a bucket, then drills a few holes from the top side and screws down into wood blocks underneath to fasten the seat securely. The seat can then be popped off to dump the bucket. Crude, but it'll work (as long as you can drill holes).