With a 40B a T5HO has to be raised significantly over the tank in order not to fry the plants.
You almost need two smaller light sources in order to cover the front and the back of the tank. Two different single bulb NO fixtures (one at the front and one at the back) might do the trick if you are talking about mounting them right on top of the tank but there really aren't many options for doing this that are both cheap and easy.
I have a T5 normal output (Aqueon) two bulb fixture that I have sitting on top of my 40B and it is too intense directly below the light and doesn't really spread to the rest of the tank. Having two fixtures like this would give good light spread but probably really high light.
There are a lot of lighting options but unlike on a lot of the common tank sizes there really isn't a clear choice for them - especially if you want to just mount a fixture on top of the tank.
The only way to do it cheap and well is to either have a light hanger or a custom canpoy that you can install DIY lights in. That is the conclusion that I have come to. If someone has a great solution I would love to hear it so I can steal it for my own use.