That is why you must use a thermostat.. so you don't overheat (although you'd have to get the wood to an awfully high temp to burn, higher than flexwatt can deliver.)
Look at Kane heat mats. They are indestructable and come in large sizes. They are washable plastic. Check out www.beanfarm.com.
You can use wood I suppose, and melamine would probably be your best bet. It seems to conduct heat fairly well for a wood product. I would look for 5/8 thick if you can find it, otherwise the 3/4" stuff can work.
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Mark G
Whitewater rosy boas, mixed rosies
Prairie kingsnakes
Cornsnakes
Mtn kings (pyro, agalma)