Hey there. I built a hood for my 15 gallon recently that I consider to be a huge success. Granted it looks ghetto as hell, but compared to the price of a "real" hood this is fantastic
1st thing to do is get the ballast/fixtures. I got a dual 24" fixture at home depot fo about 12$, and a single cheapo 24" w/bulb for 8$ so 20$ total on fixtures..
2nd thing, you need something to provide adequate ventilation so I went and picked up that white/metal/mesh closet stuff...you know what I'm talking about? Refrigerators have it sometimes its like a white grate....they had it in the closet supplies section, and they cut it for you for free. My piece cost about 3$ and I trimmed it to about 29" I think. (Note my tank is only 25" long! so there is overhang since I chose to go with 24" bulbs over 18's.
3rd...the walls....Now hunt around the wood section for a piece that is about 5inches tall, the one I got had a nice lip on it which is favorable. Of course you need to calculate the exact lengths you need and get them to cut it for you there, I think the wood cost me 4$
4th, get screws, and quick ties(to hold the ballast/fixture to the grate) this will probably come to around $4
Then go home and screw the thing together...Mine ends up looking like an A (ya know where the top of the A is flat and not pointed) the bottom area is where my filters go and where I feed the fish so I don't have to take off the full hood. Since its extra wide mine fits great...but if i were to do it again I would use those L joiner things and screw them to the wood and rest that on the tank to get a tighter fit.
so for 33$ or thereabouts I dont' think you can do much better for lighting...
then of course you have the option to get a spray can of Weather Sealant for like 3$ and spray paint for another 3$.
You'll also need an all glass canopy thing which for me was about 8$
SO-everything comes to about 47$ to get me 4watts per gallon.
I don't know how much cheaper you could do it by just purchasing the ballast/end caps...but i don't think it'd be worth the extra hassle....all I had to do was screw in a few things and blow on some sprays...took me about 30 minutes...
(oh and I coated the inside with aluminum foil.