This is what I put on over the foam and just feathered it out from there. It goes on nice and thick and is heavily textured so you have to make sure you don't get it anywhere you don't want it, but it looks perfect inside the fenderwell. Also, I kinda freaked out because it goes on brown when you first spray it so I thought it was going to look like ass, but it dries to a nice matte black and blends well.
The foam did exactly what I wanted and filled in all of the gaps so now I don't have to be paranoid about packing the voids full of rocks that will rattle, mud that will never wash out, snow and road salts that will rust, water that might find it's way into the passenger compartment, etc.