Just some suggestions

[rgilbert]

I haven’t used Hydroperm, but you can create a wax impression and use the lost wax method with it. Or you can use an alginate to make the positive, pour the Hydroperm around it and let it set up. Alginates shrink and remain fairly soft so you can remove it easily enough.

I’d Google “lost wax casting” for some insights into the process. There are also some books at your library; you might have to look under sculpture or jewelry making processes. If you’re in the US you might try local colleges or art centers for some insights.