I have same problem too and am also looking at building something but keep it in my garage which has no heat. I live in Michigan and also have cold winters for five months. I have too sulcattas and one is about 25 lbs the other about 10lbs. I have been keeping them in a 8x3 galvanized horse trough in my basement but you hit it on the head. It can smell like a barn and I am tired of that.