For optimal protection, you'd really want something that's harder to burn/ignite between the wood floor and the heat mat. And you'll probably want something moderately heat conductive on top of the mat, to both help disperse the heat better and prevent your snake from potential burning. For even more protection, however you set it up, you should consider attempting to waterproof that area with a silicone or bathroom caulk or even just a really strong foil tape or something.
My personal plan is to get some pegboard or tile, sandwich the heatmat between a couple layers, then caulk up the edges and control temp on the tile. But i haven't gotten the supplies to pull it off yet.