I've had very mixed luck with greatstuff in these. Ruined the first mold with it, had some others that just didn't skin over right.
First, do not seal it tight. The expansion has to go somewhere. Leave a small area for that
Second, some kind of mold release is probably warrented. I'ver heard liquid car wax works well, but haven't experienced it.
Third, you need some kind of mothermold (outer shell), or the latex will just deform.
Good luck, and let us know how it goes.
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