If you go to the youtube channel, some of he videos show earlier completed versions. They were made from wood, and had a simpler 555 circuit, the new ones are made from plastic, but the shape is the same.
The shells I made by first making a wood shell out of wood you can find at hobby shops called basswood. Then I made a mold box and placed the wood shell in the bottom halfway embedded in clay, then I sprayed mold release on it so the silicone wouldn't stick, then I poured the silicone over it, let it dry for 12 hours, then flipped the mold box over and removed the clay, sprayed more release, and poured the silicone again. Then once I had a completed silicone mold, I just need to mix and pour resin into it and 15 minutes later it's hardened enough to remove.
All the stuff you can get from smooth-on.com.
Here's some photos of the PKEs and mold making process though, I forgot I posted these:http://www.therpf.com/f9/my-ghostbusters-pke-meters-90889/