Sevo,
Just my two cents. I have a 32 quart rack just like yours, that has a back on it, and I ran a piece of 11" flexwatt down the back. My flexwatt is hooked up to a Johnson thermo. I keep my Nicuraguan Boas in that rack and they seem to do fine with the back heat. Also, on my baby racks, I use the same 11" flexwatt down the back, and use the reflective foam board for a back. It also seems to keep the tubs at about 93 degrees in the back of the tub, and 83 degrees at the front. My baby ball racks are run with a Helix. Eitherroute you want to go with the flexwatt, I think will work fine, and I think that the foam insulation board would keep the heat in better. Just make sure after it is set up, that you check some temps in the front and back of the tub to ensure that there is a gradient. If there is not, you may have to put more holes. With the back and sides enclosed, there is not much space for the heat to exit.

By the way, I think you did a great job for your first couple racks. Keep up the good work.