Step 3: Layout
The size of this one was based on the size of the camera sunshade, roughly 4 1/2" X 5 1/2". I added about 1/4" all the way around for clearance, and decided to make the aluminum reflector on which the LEDs are mounted 2" wide all around. The overall outside dimensions ended up being 9 5/8" by 10 3/8". With respect to the depth of the case, I figured on allowing about an inch behind the reflector for all of the components and wires, and about an inch in front of the reflector so that there would be a bit of spread to the individual lamp beams before they pass through the front diffuser. So the depth outside dimension ended up being 2 1/4' which includes the back and removable diffuser.
The lamps are arranged in two rows all around the reflector, and the rows are about 1/2" from the inside and outside edges. This leaves about one inch of space in the middle for the four battery holders, with one on each side. Spacing between the lamps ranges from about 1 1/4" and 1 3/4" with those on the outside row being further apart.
So my advice is to sit down with pencil and paper and and make your own plan. Google's software Sketchup is also great for projects like these if you are inclined to work with software.