I didn't make it any longer than the bed. I guess I could have made it a little longer but didn't have a need and wanted to retain easy access to the rear hitch.
To give a general idea on it's construction. I can give more details (if requested) later but I'm heading out in a few minutes and won't be back online until next Monday.
All wood is pressure treated.
The bottom is 1/2" exterior grade plywood.
On the exterior edges (under the storage area) is a 2x4 going from front to back. There's then 4 vertical uprights to hold the 5/4 decking boards (cut to the height of 2 5/4x6" decking boards minus the 2x4 sill plate). The top board for the storage area has a chunk of 2x4 attached to the underside of either end to keep it in place. I put felt on the underside of those cover boards to stop vibration noises. The 5/4 boards are attached to the inside and outside.
I'll have to check for details on how I made the front.
The tailgate is a 2x12 with big hinges that attach to the rearmost vertical 2x4 upright. Cables are drawn into the 2x4 framing and down to the tailgate for a limit stop (like on a truck).
For attaching, I just ran some 5/16" lag bolts up from the original holes on the underside of the OEM bed through the plywood and into the 2x4 sills that are under each storage area. The whole box can be taken off or put on in about 3 minutes.
I then stained the entire thing with Sherwin Williams Deckscapes stain. (It's a good quality stain that'll hold up a lot better than paint).
If/when you go to make it, one key measurement is the width. Make sure you make it such that once all the walls are built you still have a little over 4' in the box for hauling plywood and such.
The storage area is very handy. With the uprights it's compartmentalized for organization. I carry a first aid kit, pruners, hatchet, saw, ratchet straps, tow straps, bungie cords, a couple clevice, another draw bar, fire starters, toilet paper, flashlight, small tools, bug spray, gloves and still have a compartment for my beverages. Oh, those koozie things over your beer still fit perfectly in there.
Here's a pic once completed. Looking close, you can see the vertical uprights between the 2 boards.