> If the foam were in an inflatable pillow (or other closable bag)
> it might work to make a mold for a fiberglass seat base. Now what
> we need is a source for expanding flexible foam.

I've seen similar methods used to shape alpine ski boots to fit exactly to the skiers feet, and also seen a similar method used to shape bicycle helmets for kids with out-of-normal head shape, although in those cases they used some sort of foam that did not expand after mixing, and it hardened faster than the expanding foam used for building purposes with lesser heat productuion and it stands better to mechanical stress. I can't help you with product names, but check with suppliers of the above mentioned items.

If you should use the closable bag method, remember that to avoid touching the bottom of the frame-box or the floor or whatever the bag is placed on, the bag has to be strong enough to carry your weight without bursting.

Also remember, if you plan to have any padding on the seat, you have to use the same padding material between the foam bag and buttocks to get the right proportions on the impression in the foam.

For last: Expect that you have to sit on the bag until the foam has hardened, so remember to bring a good book, a coke or some coffee, or perform the "operation" in front of the PC and go surfing while waiting!

Good luck, and let us know if and how it turns out.

Totto