Hey Randy,
I've done that before, they work great. A couple of things I ran into probs with that might help. You need to circulate the air in them, not so much that it dries things out but enough to stabalize the temps throughout. I used computer fans that I ripped out of some old systems in my basement and then used a power supply off of one to run them (they're set up on 12V). I wired the power supply parralel to the flexwatt so that the fans would kick in when the heat did. I put 11" flexwatt on the bottom and a little bit up the sides and then put a water pan on top of it (hardware mesh over the pan just in case). I've never got around to it but I would like to pull of the original door and replace it with 1/2"-1" plex (it would need the door sealent of course) so that I could just look in without haveing to open it. Of course a good thermostat is really important. I like the GCS ones with the phone adaptor, I've got it programed to call my cell if anything goes wrong. Hope that helps some.
Sean
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Heart Mountain Herps