I helped a friend put a 250wMH on his fresh water planted tank, and in return he gave me his old 48" broken aluminum coral life fixture, one mans trash is another mans treasure. I sawed it in half using a jig saw in my kitchen, I really don't recommend that tool though because it is terribly loud, really slow and barely works, just the only tool I had besides a dremel and that was even worse.
I bought 2 300w halogen outdoor lights for $8 a piece. It just so happens that the bulb holder out of a 300/500 watt halogen fixture is the perfect size for 70w and 150w MH bulbs. They sell these lights every where, as work lights, security lights, etc. In addition the glass they use on these fixtures is UV glass and many of the 70w and 150w bulbs put of toxic UV rays that will hurt you and your tank, but the glass filter it out and UL requires MH to be behind glass in case they explode. Plus the have a reflector. (You can use the existing fixture as is and just pop in a bulb and wire up the ballast (which has to wired be within 36" of the bulb mount it and do the MH dance

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I gutted the halogen fixtures and stole the reflectors (which I modded to fit), the bulb holding sockets, and just kept the high temp wiring on it. I cut up the fixture even more to add the light where the old CF ballast used to be. I also crammed 2 18w CF ballast in next the the bulbs, in the remaining space I mounted the 2 18wCF bulbs. I made little brackets that hold the glass on but missed placed them in my move, and the glass is currently zip tied on, works great.
I wired up the MH ballast, and they sit behind the tank. The CF ballast I wired to the existing switch so they could be turned on/off independent of the MH. I really dig this light, my reef has exploded since I put it on, growth is phenomenal. I may be selling this fixture soon, since I have a 400w MH I am working on, I am excessive this light is big on a 15g, but 400w on a 15g thats what I am talking about, SPS here I come.
Here are some old photos of it: