Problem with that is that heat rises so it won't radiate down into him on the substrate. Here's what I did in a cold Michigan winter when the power went out. Put the heat pack inside an old work sock and tie it up good then pop it into the enclosure beside his warm side hide. We also did this and then popped the sock into a large margine tub with a ton of holes in it to create a sort of snake radiator for the big boa we were fostered back then (we used a few heat packs). We were actually concerned the boa might try to swallow the warm sock so this gave her heat with no risk. It worked very well to keep all the snakes nice and warm until the hydro kicked back on. We also covered every snake's tank or tub up with a blanket to retain all the heat coming off the pack and protect them from the cooler air of the room.