Take a piece of goat fencing, bend it in a shallow arc and cover with a (old style) heating pad. The older pads don't have auto shut off, control the temp under their 'cave' by altering the amount of bend in the arc which basically raises the 'roof' of the cave.
Now I need to find an old heating pad. All the new stuff is safe. I hate safety rules.
Wait, you're an EMT, probably not what you want to hear, lol. Never mind.