Take
a small piece of thin plywood that just fits in the bottom of the reinforced
BA-48 box and mount three D cell holders. Wire the D cell holders in parallel.
This
is the fabricated cover that sits on top of the D cells. The holes make
the pack lighter for those long hikes on the field exercises. Plus you can
play peek-a-boo with the D cells.
The D cell pack and the "painted" cover.
Fabricate
your 9 volt battery tray out of light aluminum sheet. If you do not have
a small sheet metal brake then use a a vice and clamp the metal between
two pieces of wood.
Half inch oak blocks form the ends.
Secure
the ends with wood screws. 
Holes
were drilled in the sides to make the assembly lighter.