I'm tellin u this now so u dont have to deal with it later, heat is going to be a problem for you. I had the exact same dimensions with a 400 watter and it was in the high 90's all the time with a sufficient fan pulling air out of the box, the key to doing this is going to be a cooltube or something similar, and venting somewhere the heat has to go. I wound up destroying the box cause my plants were getting to hot and just outgrew it.