I've got one shoplight over my 55g, on a wooden square frame that I built. I'm going to have to upgrade the tank to a 75g, as my 55g is very old, the center brace is broken and it is slightly bowing...but that's for another discussion.
Anyways, when I do it over, I'm going to make wooden frame for the shooplight a bit differently in order to reflect the light better (right now it's just a frame with 90-degree angles) and put a couple center braces in it (in case a bulb slips out...which they tend to do on shoplights).
At this point, I've retrofitted the shoplight with electronic ballasts for ODNO and am very pleased with the results...so I'd rather invest in some shoplight retrofits to make it more suitable for use over an aquarium. For example, you can buy waterproof endcaps too, which is what I plan to do as part of the migration to a 75g tank.