I used to pre-mix the soup and carry it in a plastic lunchbox- you know the freezer type ones with an airtight lid. Mine doesn't leak, even when it's bouncing around in a back pack, although I'd wrap it in a zip-loc plastic bag for safety.
I'dd pull the 'roid, throw the neg in, and seal the lid back down. When I ran out of film and have about a dozen negs all swimming around in there together, I'd take it home and wash them down. No problems with scratches either. The sulphite keeps well in the air tight container and you can use the same brew again next time, until there is so much Polaroid gunk floating around that you're afraid to get your fingers in it.
This is all written in the past tense, because lately I simply shoot on transparency film and then pop the processed tran in the enlarger with the 5x4 pol-back face up on the base plate. Then I can process the roids in the comfort of the darkroom
Is this cheating