Dude for real, after all the work of putting everything together you're going to risk it by putting a piece of baking material between it?
Get a gasket dude, i'm sure bob at SDD can help you out. Shoot him an email to Robert@SinisterDD.com or visit their website at Sinister Drag Designs - Chicagolands new performance specialist! I'm sure he'll agree that baking sheets aren't going to "cut" it (LOL i crack my a$$ up)