Welcome to the site Neil!!!
I think you can use anything you like for transforming something into a master...styrene is the material of choice, but diecast pieces work well, fiberglass, just about anything that'll stand up to the mold-making process.
You mention "non-hardening" clay? I guess as long as you don't obscure or disturb the soft material during the rubber pour, you'd be OK. But a durable master is preferred, then it can be used again after the first mold wears out!
Show us what you built!