Boinggg!!!

The derating usually works like this:

1 unit, 600 watts
2 units, 500 watts
3 or more, 400 watts

So, you are actually exceeding the ratings a tad on two of them. The little-but-useful idea might be to place the 3-fixture unit in the center.

You could always use a steel plate with a thick smear of conductive goop behind it, and maybe drill a few 1/4" holes in the top and bottom of the box. Watch the wires, both inside and outside the box!

Larry
www.fineelectricco.com