well the whole idea of a shallow tank is to minimize the need for higher wattage bulbs. i am thinking a rubbermade container (that will fit your design) filled to 12 inches deep would do the trick. anyways i don't see the need to go MH if you keep the tank shallow (14inches max) and the lights fairly close to the surface (with enough care to prevent splashing, etc) i could have swore garf.org had pics of their frag tanks, i even remember reading an article that said they were rasing coral frags on 2bulbs @ 40watt - shop light fixtures.
i will try double checking garf.org for the articles i am thinking of (couldn't located them just now). but the shallower the tank the less demand you have on lighting, since light can easily penetrate the tank.