I have four Philips 40w TLK/05, 60 cm long actinic tubes (350-400nm UV range) and the necessary accessories to connect them (ballasts, etc.).

I'd like to use those to build a 50x60 exposure unit (60cm is the lenght of the tubes, 30cm is the maximum lenght of the shortest side of the negatives I can produce, while my contact printing frame covers up to 40x50cm sheets).

Since they're only four -but more powerful than the common 8-12w tubes used- I'm a bit concerned about the distance of the printing plane and between the tubes. Considering that my largest negative will have a 30cm side, I calculated that placing tubes at an even distance from each other this distance won't be shorter than 4cm. As for the distance from the printing plane, I don't know the rule, but how about 30cm?

I was also thinking to paint white the inside of the box and perhaps put an aluminium plate under the tubes. Are there other tricks to enhance the diffusion of the UV light?