I built a UV box using spiral BL lamps that works just fine for pt/pd printing.

I used six bulbs spaced on 6" centers (two rows, three bulbs per row). The height of the box was designed so that the space between the surface of a printing frame the bottom of the bulbs would be about the same as the center-to-center spacing. I cannot detect any variation in illumination with this spacing.

I put a laid a sheet of aluminum flashing across the surface to act as a reflector before mounting the lamp bases. I don't know if that was necessary or even if it is of value - it just seemed like something that made sense, and I had a roll of flashing left over from a previous project that I could use.