I would be careful about casting inside the piece as some materials might expand or shrink as they cure and possibly crack the piece. If you want to make an interior, you could possibly use play doh to make a positive mold of the interior and pour plaster around it, or simply carve some details in it, then let it dry outside of the piece. The lightweight airdry version wont crack as much when drying as the classic playdoh.
Beautiful finds. A little flock and plant material will make some wonderful jungle temples.