Ghetto blinds...

I put nails along the top of the door jam, and hung an old blanket on the nails. A dark comforter or a couple of blankets work, too. Just be sure the wife isn't looking forward to using them next winter, tho. It wasn't using them that got my wife all tweaked...it was the holes I had put in 'em.

I ended-up taking individual links of a 'S' hook type chain, and fastening them every 6" or so along top edge of blanket. (crimping them to the blanket) Then you can unhook them from the nails at will.

Keep in mind which way the door opens, and whether or not the blanket would hinder entrance. Step inside once done, and see if any further adjustments are needed. (no leaks, not near the lamps or fans...)

I used this set-up on a 4' x 2' slider-door closet for a couple of years. Ventilation (consistent fresh air) is critical to keep in mind, too.