While I do have a DIY light fixture (T5HO/MetalHalide combo), I don't really have a build thread to help you out. I'll provide a few details here though & a photo or two. I'm currently building an LED fixture to replace this rig & would recommend you look into LEDs as well. They are well on the way to becoming the standard for aquarium lighting & prices are coming down rapidly.
Anyhow, my tank is 4 ft long 77 gal Hagen & I started with a 3 ft Hagen GLO two lamp T5HO fixture. Quickly realized this was not going to cut it for where I planned to go with my system, so set about building my current DIY ghetto rig. I cannibalized a used MH/VHO fixture for the Workhorse 8 fluorescent ballasts, purchased water resistant endcaps online, aluminum stock from Princess Auto & some 12VDC fans since I'm overdriving the two T5HOs I'm running now. Before adding the 2x250 watt MH lamps, I was running 2 sets of 48" T5HOs using one Workhorse 8 per pair. After adding the MH, I retired one set of T5HOs.
Standard T5 lights are not going to cut it for any kind of saltwater reef application, so the only real option will be T5HO, which require a suitable ballast. An F24HO (24 inch) T5 lamp will use approx. 24 watts driven at the rated power.
The whole rig pretty much as it is now. I also built the MH reflectors from a pattern I got online & sola tube reflective aluminum from major hardware store:
Close up of the water resistant T5HO endcaps with cooling fan. Also note the high quality reflector to maximize light:
Hopefully this will give you some ideas of how to get started. With many folks switching to LED, there should be plenty of used T5HO parts & whole setups for sale at very good prices. But I'll reiterate that you're probably farther ahead going LED right away yourself, particularly for a tank your size.
Here's a link to my LED project. I haven't done much lately since I've been away skiing the backcountry. Hopefully get going again now that I'm back.
http://www.canreef.com/vbulletin/showthread.php?t=82206