John dunno know if you already used but the Mori Mori polyester putty seems a good deal there: it cure 100% in 24h (like a rock) but just after 20/25 mins you'll cut easily with a knife and sand, and it wouldn't lost volume. You'll fill with polyuretane foam from a couple cm from the base and than with the Mori: it's awesome (but smell very very hard). I tried also some automotive poliester's putty but they haven't the "step's curing" of the mory: their curing is linear, like I said the Mori transform its consistency and after a couple hour start to be almost rock.