I can't see how of the shelf canned expandable foam is going to work. It appears to need a good air supply to cure. I found that it does not go off in a bag and as soon as you put pressure on it, the foam turns to a mushy resin.
I'm going to have to move on to the 2 part foam from demon tweeks.
I found this link, with some good pictures.......I think this car is more about economy than speed though !
Another moulded foam seat link