It's funny, I was just outside building something for my two windows in the bedroom earlier today. My bathroom has just gotten too cramped.
I used what I had, which was some 1x1/2 wood, and made a square frame a few inches larger than my windows from 4 pieces (8 for the 2 windows) and some wood screws. Looks like a canvas stretcher. Then I braced it in the corners, and found some old boxes that camera gear had come in. Those boxes were broken down and cut to size, then stapled onto the frame. Then I painted it w/ flat black from some spray cans. It uses up a lot of paint though, and the coverage is not good, so the other one will get the paint brushed on.
Next, I am going to cover the edges that sit against the wall w/ some old cut up towels or felt (just staple it on) so there will be no light leaks on the edges where it sits against the wall. I screwed a 1x1 the same length as the frames under each window to rest the frames on, and am still figuring out how to quickly and easily cinch the frames up to the wall. Might just use a couple of bungee cords to pull it up flush once I set a couple of hooks on either side of the window. All I needed for tools was a staple gun, a Phillips bit for my drill, a handsaw, and a pair of scissors for the boxes.
I had been duct taping foam panels up to the window in the bathroom to keep light out, and it was a major PITA to undo it every time. Plus, foam board is not good to use, as it will allow a lot of light to shine right through, so I had to paint them anyway. The cardboard boxes are better at stopping light. This new system will be much faster to put up and take down.
For your door, just use a thick blanket. Stuff it in at the top (or tack it, or tape it up) and it will fall flat and only need a little taping in a few places on the side. A rolled up towel is good for the bottom if your blanket doesn't go all the way down.