Grats on the pied. I love pieds.
The rack should be heated with flexwatt. You need a temperature controller to measure and control the heat. The temp controller probe will be placed behind one of the tubs in the rack to measure the hotspot in your tubs. If the ambient temp is not high enough, you may need to heat the room that the rack is in. I use oil-filled radiators to heat my snake room. I suggest purchasing a temperature gun to measure your temps. You can buy them for around $25. They are great for snake owners who have to measure more than one enclosure.