I use foil-backed foam insulation board to plug up windows - available at any home center. It's easy to cut with a simple knife, and deforms a bit to wedge it in for a tight seal. Cheap. Easy to remove, does no damage. I like to use the foil tape on joints.
I've never done a whole doorway but don't see why it wouldn't work. With a minimal amount of support, I could even see building the walls of an entire small temporary darkroom with the stuff.
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