Hi, I actually use a growtent as my portable darkroom when I make collodion plates outside. They are quite perfect! They are light-tight, have room for fans, mine also has ventilation holes and things where you can attach a red LED to as a safe-light. I bought mine from this company:
Dead Link Removed
Here's a shot of mine:
The only thing that's tricky is that the zipper is being handled from the outside, not in. This means you can't close it all the way down but if you sew some black cloth at the bottom part where you can't manage to fully zip it you should be more than fine.
I recently saw an ad for a darkroom tent for children to play in. Maybe you can find something like that, it has zippers from the inside...