If ya want to make a symmetrical piece, could you make one side then use that to make a mirror image?
MMMMkay, Looka whut I done gone & found
https://groups.yahoo.com/neo/groups/casting/conversations/topics/31013Ive seen guys build fenders on a French wheel. How they are able to make that second fender come out as a mirror image of the first is just amazing to me.
Anyway, look up making a mirror image mold & see if theres anything you can find there.
Another approach may be build one side then build sections of the other side & put them together. If an angle isnt perfect, shim the sections to accommodate that & fill the gap with, well,, play doh if ya hafta. Heck, modelling clay is out there..
You dont wanna mess with bondo on foam,, Holding the piece down
to sand it wont be easy,& unless you have experience in sanding bondo, youre just not gonna get it.
Heres a starter clue.
Say you have a Ford pickup door panel & youve got Bondo ( "mud" for future reference) from the door handle down, front to back & you can feel a high spot that drops off harder toward the front of the door & tapers off more gently toward the rear. You take your long sanding board & sand, pushing the board from the top of the high point toward the front of the door. Lots of people want to start sanding where the mud is closest to what they want & work toward the high point.. thats not gonna work.,.
So,, maybe modelling clay on a foam block & a piece of plywood to lay the piece on, some wire to put on the wood & hold next to the part & compare heights at distances from center..
You mite get a broom stick & make a few "legs" & mount them on the plywood & put holes in the foam part way, drill down, put a fat washer in & mount the block to legs.