don't just go with a square box. You can lose space on the top corners where the reflectors curve. You'll make it a trapazoid and it will look 'lighter' (sorry for the pun)
the other option is to make it from metal. aluminum L bracket is cheap, so is aluminum sheet. it's just as easy to work with as wood.
Remember that the bottoms of the reflectors should be even or your gonna get a shadow where the halide reflector is. So the piece is gonna have to look like:
____
___/ H \___
A A
If that makes sense. thats going to be the shape of the bracket.
It its going to look silly with retro reflectors hanging off a metal bracket with screws and bolts from home depot (although the concept is nice) so plan on finishing it nice with small screws and get your dremel out and ready to grind stuff down.
Your also going to run with balance issues with the power wires pulling on the whole fixture. Your going to have to run 2 wires from the ceiling just to fix it and flare those out as they meet the bracket to counter balance it. And no matter how well you do it its still not going to be centered as nicely as you want it.
Also, your gonna love it for about 2 weeks till the light spill out around the tank gets to you. Then your gonna hate looking at your tank with the lights shining in your eyes.
IMO it would be alot easier and less toying around by building a wood hood or something. =0) Just my experience with the whole hanging light thing. Hope it helps a little.