Thanks for your help Kaorte and George. I have been using a heating lamp thus far for my crestie. He finds his way up under the light more than you think. But the real danger is forgetting to turn it off. I usually just turn it on for an hour or so to get his tank temp at about 75 (that's my goal). Sometimes it's lower, sometimes it's higher.
I imagine there won't be too much circulation if I am using a plastic tub like this (thus the cut out on the side). But it relieves me and worries me to not have to make a cut out on top. It relieves me because the enclosure will be safe and easy to make. Yet, I worry about it getting too cold for him? I guess the danger for cresties is extreme hot, not cold? Thanks for your input thus far and anymore you have.
I am going to start this project this weekend and I will post some photos, assuming I find it presentable. Thanks!