It is hard to tell what your photo shows. There is not supposed to be a void under the mast or above the compression post. The whole path between the bottom of the mast and the keel should be solid. It there is a void in the top of the cabin, it would probably be best to fill it in with plywood that has had the edges epoxied to stop water ingress. Foam would be too soft and spongy to serve this purpose.