When I built my UV box I installed a 4 inch fan I bought at Radio Shack and it has worked for years and years of printing. One thing to consider if you're building a hinged box, ie the bulbs are in a box hinged to a vacuum frame, is to place the ballasts with a fan in a separate box since they comprise the vast amount of weight in the box. That makes the part you have to lift much lighter. Unfortunately, I thought of it after I built it.
Consider, too, the type of bulbs you install which should be specific to the process. As reported in many online groups, Pt/Pd prefers higher nanometers (nm) than other processes. Many people and books recommend your typical "black light" bulbs which generate most of their light around 360nm while Super Actinics output a narrow spectrum near 415nm which results in faster print times for Pt/Pd. A friend of mine who prints with POP compared her UV lights (black lights) to mine (SA's) and found mine printed faster and with a small, but appreciated increase in contrast. I don"t know other processes and what spectrum they prefer, but maybe others on this group do.