I have had good coverage and uniformity for 10x10 placing 9 13W spiral BLB bulbs within a box not larger than 28x28" with a height of 6" from the bulb bottom to print top.

The inside of the box is all white.

Sides and top are melmamime particle board, as I recall, because I built it from the remians of an old cheap Ikea style desk I had disassembled and was redy to take to the dump after getting wiggly loose one tiime to many.