I never put together a very detailed write-up but once you get out a tape measure and start mocking it up, figuring out dimensions goes fairly quickly. I started with a simple box, 42" long, 41" wide, 13" tall. This is the height I picked to match the folded seat, then max width between the wheel wells, length is based on middle seats to tailgate measured at 13" high.
After that you trace the fill-in pieces between the box and the outside trim. Next make drawers that fit in your box. All custom based on what you want in the drawers. I used some common items such at my trusty coleman stove, typical suitcase etc to determine drawer width. Paul's is extremely professional, everything built in from the get go. I myself went with a simple design and have been slowly adding to it as I figure out what we need/would actually use regularly.
Building drawer is a very rewarding experience. Significant time, low cost. Opposite ratio of the 1500 front bumper that is expensive and installs in about 2-4 hours. Drawers were about 200 in material and 20 hours of measuring/cutting/gluing/sanding etc.