Paper meache'. (hope that's spelled right). I built the following in Art Class my senior year. The downside is that they really don't last long as far as maintenance and sturdiness. But at least I did give one heck of a show on lip sink day.

-Ghost Trap (not used in show)
-Proton Pak
-Ecto-Goggles
-PKE Meter
-Ghost Sniffer (not used in show)
-Slime Blower (not used in show)
-Ghost Buggy (smaller model, only Filmation prop made, not used in show)

Interesting, got any pics of any of this? Such as progress or completed projects?

It's spelled: papier maché and indeed, it doesn't last long, nor is it suitable for smooth textures…