Urethane foam is really sticky stuff; it will stick to just about anything. I've never cast it into anything but silicone rubber myself. You could just carve your form positive and make a silicone rubber mold for casting parts in foam. But if you want to carve the mold instead of the part, do a test in a couple of materials that seem promising - like machinable wax and LDPE. Just make a little cavity in each sample, then pour a little foam in it. See what happens - and let us know, okay?
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