Foam is awesome. I made a rudder for a Kit plane using Aerospace Foam, and fiberglass. It was unbelievable strong, two people could stand and bounce on it between two cinder blocks, with no damage, ( had cloth on the blocks), As long as you have a way for the foam to escape, it could be used in virtually any paper model. This would allow for sanding a surface, in which you made the cavity over sized, and allow the forming of compound surfaces which are part of the model. It is then covered and smooth to the desired texture with filler. It has to be sand-able foam though.