Do a search for Screw-in UV bulbs, or spiral UV bulbs on APUG and you will find some good communication on building with screw-in bulbs. They are working well for many people and I calculate the cost to be a substantial saving over tubes requiring ballasts. I am experimenting with one spiral bulb in a #2 Photoflood reflector as an easily portable source for demos. It works well with 4x5 and I believe it will do so with 5x7. For 8x10 I would look at 4 of these bulbs as close together as the porcelain bases will allow.