I've got one of these. Not for ice fishing, not for growing pot, not for a darkroom, but as an outdoor weatherproof place for fusion splicing fiber optic cables. The windows and door button up good and it would make a decent darkroom. Its pretty well made too.
Using those grow tents would be a bad idea, they're totally reflective inside. So unless you're only trying to reproduce gray, i would stay out of them.. unless you plan to cover the entire inside with black fabric... that wouldnt work wonders for dust though.