I have a 55g and 2 shoplights. I suck at the whole measuring and geometry thing. Oh yeah, and the whole building something substantial and study thing.
If you want to put a shoplight into a hood its pretty easy, depending on the mounting options of the light. If you can hang it with a chain, just put a couple hooks in the top of your canopy and hang it from inside the canopy. I took a strip of wood and with the assembly dissasembled, screwed from the inside of the fixture through to the wood. The wood strip is perpendicular to the fixture so it protrudes from the fixture far enough to get a drill in there and screw it into the inside top of the canopy. You could screw it right into the hood itself, but I wanted to leave an air gap around it and get the lights a couple inches closer to the tank.
You can lay regular flourescent right on a glass top or plexi since they dont really get that hot, but a PC fixture gets pretty hot and would have to have legs to stand up a bit off the surface. I ran 2 twin tube shoplights over my open tank for a while but wouldnt recomend it due to the possibility of problems.
I built a canopy with shop lights this past weekend...next time I will make cabinet doors rather than a lifting type lid as the suggested methods show.
just an update, but I went with the ebay hood after deciding the DIY route was too extensive for me. I don't even own a hammer! I worked some extra shifts overtime, bit the bullet, and spent the money. I do have the hood up on feet which raise it 3" from the top of the tank. The plexiglass sheet "covers" do warp slowly, but it's no big deal since I just flip them once a day. Overall, satisfied with the lighting system...I've even dropped it once in my own stupidity and it's survived that
Yeah, DIY is usually only cheaper if you already have most of the tools. Sometimes we get lucky and already have most of the parts hanging around too. But if this is not the case, ebay, and some other cheap online sources is the way to go (dont forget about craigslist for used stuff).