[Big Dave]

Looks good. I like how it tapers in the plan view.

My old plywood and rubber apparition looked a lot the same. But you have a lid and that makes it much more practical.

I would offer an improvement. Don't carry your cab bulkhead section so far back. 6 inches max. then begin a smooth transition into your slope. Also you can more closely approximate the Hucho template. Also this approximation would leave the back of your slope about 8 inches above the top of the tailgate. While this results in a bigger wake area (bad) the flow stays attached to the slope resulting in less drag. Bondo made his beautiful fiberglass top that way and in the wind tunnel the smoke stuck to the lid all the way to the end. A higher rear lip allows the owner a wider array of freight - always important to pickup guys.

Get with Phil Knox (aka aerohead). He's forgotten more about pickup truck aerodynamics than most of us on this board will ever know. His crazy (but probably effective) ideas scream out for somebody with the manufacturing smarts your work evinces.

Depending on material costs, you design looks economical. Maybe you could even design it to be molded out of ABS. ABS commits you to high-volume tooling. The stuff is not easy to fabricate, but makes a good product. That's why it dominates in automotive accessory bodywork.