The FOX "blank" boxes were purchased from Par-Metal
http://www.par-metal.com/

We used a "create-a-size" chassis that is
19 inch wide x 24 inch long x 3.5 inch high
with their standard 1/8 inch front panel thickness
and unfinished.

The part number for the boxes is 16-192403-U 
and the dimensions can be found in the par-metal specs here
https://web.pa.msu.edu/hep/atlas/l1calo/fox/hardware/components/chassis/