If you don't want to use fiberglass then I'd just get a bunch of epoxy and high density filler and mix up the epoxy to whatever consistancy you feel is best (I'd vote for "peanut-butter"-like) and build it up that way. I suppose if you thought you had a LOT of buildup to do you could get some dowels or something like that to take up space and put the thickened epoxy over *that*...