This is a very common problem (also hoods are crazy expensive, like, more than the tanks... What's up with *that*?), but there is a very, very easy solution.
The easiest way to do it is just to have some plexiglass/acrylic cut to fit the top of your tank. Then add a clip-on lamp to the back, and you're good to go.
Most hardware stores sell acrylic/plexi, and some will cut it for you. If they won't cut it, it's not hard, you just get a plastic cutting knife and a straight edge, and they should show/tell you how to do it.
Just make sure to cut it so that there is some overhang (so the acrylic fits over the top and doesn't fall in), and make sure to leave a space along one side for air to come in and out, and for your lamp to clip on.
The plexi should cost you about $10, and the lamps are about 7-10 as well, so your entire top will only cost you about $20, far less than a hood, and you'll have some plexi left over for additional tanks as well.
Here's a photo of two of my tanks done this way:
And here's links to finding the lamps:
Do you have a walmart
near you? Here's the link to them there: ($7 at my local store)
Or, here are some from amazon
: ($10.00 with free shipping at 25$)