After reading the Benson book, I wanted to get hold of the material Celastic, as it sounded like a perfect material to use when making gaffs and prototypes.
Not that easy to find. But I found something even better: A non-toxic thermoplastic resin called "Varaform". Just heat it up in water, then make any form you want - and as soon as it cools it is rigid again. Just google for the word "Varaform", and you'll find plenty of suppliers.
Here's one place: