I'm doing silicone sheet gaskets:


Along with some other Bonneville guys:
BonnevillePRO.com • View topic - Silicone sheet to make reuseable s/c / IC / LIM gaskets?

Not using a baking sheet, but rather some silicone sheet material:
Silicone Rubber Roll and Sheet for Gaskets

As long as the baking sheet is somewhat smooth (no deep "pattern" molded in) and is large enough (minimum 11" by 15") I would assume you'd be OK.