I'm trying to figure out how to make what is effectively a booster seat for the back seat of my car. The back seat is on the order of 42 inches wide, and I would want to make something that was about 18 inches tall and 16 inches deep. It doesn't have to be super durable, but it needs to be able to hold two not-terribly-small people for about half an hour of smiling and waving, but it needs to be reasonably stable. Not damaging the car is also important.
My first thought is to use an expanding foam to mold something. I know that this stuff is sometimes used for creating custom race car seats, so it should be workable for this. I could line the bejeezus out of the back seat with plastic tarps, use a board along the front edge of the seat to get the right depth, then pour in some expanding foam. When it's cured, I'd trim the top of it to the right size, and then cover it with more plastic.
Other ideas? Something with upholstery foam perhaps? Or something beanbag-ish?
I just did a compression test with styrofoam, since I had some sheets of it sitting around. I stood on one foot on a 3" styrofoam sheet for one minute, and it compressed less than a quarter of an inch.