I have a couple in my build list right now. My thoughts were to resin it on the outside at least 2 coats, pour a batch of "thin" rondo on the inside, then fill the remainder with foam. When I say "thin" rondo, I mean runny, probably more along the consistency of a milkshake than pudding like usual. With the rondo and foam method, you should, ideally, be able to do everything through a somewhat small hole somewhere in an inconspicuous location. It should take less effort in the finishing process. Just my two cents, hope it helps at least a little bit.
-Matt