...suggestion #1: make sure your grow box fits where you intended it to, and you might try re-drawing it to scale. The basic design is good, but before you build it, try finding the rubbermaid tub you want, and then make sure it fits your design.

Suggestion #2 would be to add intake and output fans, and to shroud 'em so that they don't contribute to light leaks. Look into 120mm 12v computer 'muffin' fans for this task. Also, think about adding some kinda cheapie furnace/ducting filter to the intake, to help keep the atmo pure in there.

Suggestion #3 would be to determine what type of lights you wanna run (CFL?), and figg'r out how you're going to stack 'em in there. Again, try to make sure that your lighting is workable before you build the box (you might find that you have to make it a bit bigger or wider or somesuch.

Suggestion #4 would be to make sure your 'doorway' is workable, and not going to cause you pain. If you're going hydro, you might wanna make the 'door' a whole 'side' of the box, instead of the half-side, half-top thingy, just so that you can insert/remove a rubbermaid tub full of water without logistical nightmares. Even dirt-based plants can be a pain to mess about with

Suggestion #5 would be to HAVE FUN, and to persist in the face of difficulty! (and keep smokin'!)