Another thing Mego was famous for was slightly re-dressing figures and accessories to become "new" ones as needed. I think maybe I'd have done a repaint on the Bat-Signal that came with the cave playset and made that the Phantom Zone projector.
Sure, it'd be a bit big, but Mego wasn't famous for scale, either. The Batmobile was so small it looked like it should bounce the Dynamic Duo up and down when Batman put in a quarter.
As an aside, this customizer also produced a pair of terrific Nightwing and Flamebird customs, complete with a "blister card" featuring Curt Swan graphics.