I too would be concerned that the snake burrows into the substrate. Flexwatt is designed to have air circulation on at least one side so it doesn't get too much heat built up. I would be very very cautious about sandwiching the flexwatt like you have described. Perhaps a solution would be to wire the flexwatt parallel and add a rheostat to the sections that you need to reduce heat to that is a safer alternative.
After everyone's input I'll be putting Lactose back into his tank I do believe.
