Hmmmm....I'm not sure of the toxicity of fire retardant but I would imagine that the safest thing you could possibly do is get melamine (I hear that melamine is pretty fire resistant). I personally wouldn't expose my snakes to DIY treated wood because I'm not a risk taker like that. I might just be crazy though and it could be completely harmless because I don't know too much about what type of product you are going to use to treat the wood.

On a side note: nothing is going to be 100% fire safe; you're always going to have the potential of fire when dealing with the amount of electrical aspects that goes into BPs. But also, in the pursuit of avoiding a pitfall (fire), don't unexpectedly jump into another (poisoning via toxins).