Trial and error! I made 7 skulls for him before getting one with the right shape. It had to have the long, round-ended schnoz, short lower jaw, round cheeks, and overall flexibility of the original puppet. Oh, and the puppeteer's hand has to be able to fit inside it too. That was a tough combo. At least I got a bit of a hint from one of the DVD special features, when they talked about Wem's mouthplate. It's not two separate pieces like most other characters' are, but one piece, with the back scored so it can flex smoothly.