just a little thing, but do you really need to have it shipped?? -sry just seems weird that you don't have a closer art supply?
-well anyway, it may be a bit beyond you in price range at the mo, but have a look at rubber moulding compounds like "gelflex".
it comes in two colours (white=soft/ blue=harder £6 per kilo here, but cheaper in bulk) http://www.tiranti.co.uk/indexhome.asp
can easily be used to make very complex finishes (and will have little problems when it comes to undercuts) all you need to do is melt it down and pour over. this could be used as is (if it is thick enough to hold its own weight, without deformation)
in your case though, i would maybe look at doing a thin layer (2-5mm) and backing it with a plaster/hydrocal jacket (just remember your locating pins!)
this would mean you wouldn't have to worry about the fast set of the hydro -you would build it up in a similar way to using brass shrim to make piece mould, you just have to file the surface back to the join and then gently open like you would any other mould
hope it helps