I built a wooden cage (you can see it in the DIY section) and I'm heating it using a sheet of flexwatt inside the cage.
The wires run outside the cage and there is a ceramic tile on top of the flexwatt. On the tile is about 2-3" of substrate. The max temp I'm gettin on top of the substrate is like 94-95 degrees now depending if the door to my room is open or not.