A friend of mine (really, I'm not being "online coy"  ) has been trying silicone baking sheets for wrapping the mix ( "cutting some holes in it, folding in four, then putting the mix in the cavity, sealing the top and bottom with uncut sheet and weighing it down with a pyrex lid in the oven" )  and says the sticky end result comes off really easily after baking.

That sounds like a pretty airtight method, I'll let you know how they get on