Hey Readytoride08,I personaly have never had a rubbermaid grow box but,for the light situation I would add more lights than with the 20 gallon as well as a higher CFM rated fan(being that you are basicly doubeling the size of the grow area)just to be safe,more light and air won't hurt.As for leaving the box open,if you are going with the original set up (one on top of the other) I don't see how you could leave it open being that one supports the other.If plant hight is a concern either go with a Indica domminant plant or do my personal favorite and LST her,or do both.good luck with your grow!!