I believe the standard practice is to use a piece of natural cork, such as one from a bottle on wine. I got a bag of them at a local craft store, inexpensively with a coupon. The wire armature sticks into the cork quite well if you leave a bit of extra length.
I've never heard of the cork burning, at least not at ~15 minute bake time.
a wooden board will do. the kind of time and temperature your looking at shouldn't cause a problem, if in doubt dampen the wood before you put it in to slow any potential charing (most poly clays i've tried cure around 110-130 celcieus so it shouldn't char anyway). drill holes for the armature wires