I have been using the heating pads you put in a green house to help start seedlings. They are waterproof and stay at a low but warm temperature. There are also the pet-store mats (you attach to the underside or side of the tank/aquarium--I do not use either). But my torts are still very small. Thank you for the recommendation for the pig blankets-- I will use them as the torts get bigger!
In the enclosure I use, I have put substrate on top of the pad and they have enough room to get closer or farther away if need be.