I need help constructing an indoor iguana cage that can hold constant humidity of 60% in California?

I have a one year old iguana and need a cage that can hold it 4 years from now. i live in southern california so maintaining a constant humidity level is difficult. does any one have any ideas on how to build a cage on a budget?

How do I build a iguana cage?

Does anyone know where I can find some instructions to build a cage for an adult iguana? I'd perfer if they were free online instructions please.
I'd need something closer to 6ftHx3ftDx4ftL