[wilfong9]

I'm not sure if yours is the same as mine, but on the 08 there is a seat belt assembly that must be taken account for. What i have to do on mine is make the back of the box left side 12 inches and the right side (closest to the center of the truck) 9.5 inches. This leaves the center recessed part 7.5 inches wide for both the belt female recievers and the assembly attached to the back wall.

In your drawing the C's would just be 12" and 9.5" and the E would be 8.5" if you are using 1/2" MDF. Don't know if this will help anybody else but just something i noticed.