I'm in Kansas and I don't have a roof on my outdoor pen. It's roughly 3'X 6'X 27". That's the size 3/4" plywood I had that I used for the base. It holds three bags of cyprus mulch(Home Depot). There is a large water dish and a hide. The tegus usually just dig in for the evening whether it is raining or not. I built the sides with a little gap so it would drain. So far it has worked fine. For the most part I don't think the rain hurts them because the temperature stays high, and proabably will, through September and into October. I am shooting for the third week of September maybe later to move them inside. Hope this helps.
Mike B