Office supply and crafts places sell black cardboard backdrops for science fair style displays. These are tri-fold and free-standing and should wrap around the back and part way around the sides of the enlarger. The black ones I've seen are pretty flat textured. This would block the mirror, be fast to set up, and easy to stow behind a desk, dresser, etc. when not in use. As long as the height is good for blocking the mirror from the enlarger head and/or easel, I'd think it would work well. Cost is about $4 or so.
Lee