i built my hood about 7 months ago, thoughts : design so the front opens up. not just the top. this is a little harder but makes life soooo much easier on you, if you are planning on doing MH then your hood has to be atleast 10 inches tall more like 12 if you are reaching all the way down the top getting access to things on the bottom becomes a problem. also use stainless hinges (get them at a marine supply store). they cost more but brass or aluminium will be toast insde a year.
another thought, fans. you are going to need them. figure out how you want the air flow to go across your bulbs.