My brother has an ig, and has actually raised several successfully. I can tell you this, a 20 gallon tank is not big enough for yours, currently, my brother is housing a 27 inch ig in a HUGE custom wire cage (they do better in them) 3 foot from side to side and 3 foot from front to back, and stands 6 foot tall. Yes it may be a bit big, but at least he has plenty of room to move around. The more space the better. And the more climbing branches and stuff for climbing the better. They should have plenty of different heights from the basking light to choose from.

Also, out of curiosity, what happened to the other one you had that got big? Do you still have it?