I built one myself for my 40 inch and growing alligator. I made a 4'X 8' with a 3' x 4' sunning area and a 4' x 5' swimming area. I framed it up with 2" x 4" studs and closed it in with plywood. I lined the swimming area with a pond liner and heat it with an 800 watt titanium aquarium heater that I bought online. I bought a pond filter and pump and it all seems to be working fine for a couple of months now. It is in my basement and I use a dehumidifier to take the extra moisture out of the air to help prevent mildew. I'll send you pictures via email if you want, I don't have a hosting site to post them.
Mark