A friend has done this and swears by it. He does say that he has to replace them every year or so though. I tried it with 2-part foam from TAP Plastics and decided not to use it. I had trouble getting it to leave any space under the lowest part of my butt and in getting the forward part of the formed seat in the right slope.

If you try it, I suggest you start with a relatively small amount of foam and work up if that isn't enough. It does generate some heat, bearable in the amounts I used, but I can't imagine what whould happen with a large volume. Also, be careful about leaks from your garbage bag (use trash compactor bags, they're much sturdier). The foam sticks to most stuff, including legs, and prying globs of hardened foam from a hairy leg is not a recommended pasttime.