Hey Blackula, what you could try is filling one half of the mold to the brim, place the other half on it, then kind of rotocast it with only half the casting resin you would normally use (the resin in the first half of the mold)
That way you cover the surfaces, but theres not enough to make it solid.
Kind of understand what I mean?
It'd be kind of taking a risk though, because you might just miss a spot since you can't see inside the mold. Yet considering how small the piece is, that's not very likely.