use cardboard and masking tape to reinforce it if you make a large part. Just make some narrow 1/2" strips of cardboard or paper internally to help it hold it's shape better. Once you get it formed you can sand it carefully then cover it with a layer of lightweight fiberglass body filler then paint it.
I have patched body panels on cars like that and it actually works really well. You can use a high speed sanding wheel to shape the foam a bit lower than you want it then cover it with fiberglass body filler. Smooth the filler like any other body panel then you can paint it and it is perfect looking.
Also it is nearly impossible to keep the nozzle clear after you use it so once you start using it, make all the parts you want. It will probably be clogged up next time you go to use the can
