The regular spacer the factory installed was Bakelite, a primitive form of phenolic. I've seen home-brewed ones made from various kinds of plastic, wood, and other non-heat-conducting materials. Whatever you choose, I doubt if it would be more effective than the OEM reproduction spacer most of the restoration places have to offer. Be sure not to block any vacuum passages if you choose to make your own.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!