I'm not a fan of Coroplast, I mentioned it to fit in with my peers. I like redwood benderboard for framing. You may have seen this:
1/4x4". You can rip it in half for skip sheathing for your canvas, else stack pieces and taper them all at once so you can skip the canvas.
I guess I meant cargo carrier mount not hitch mount. What is it, about 2ft long? A 2ft truncation will give about half the drag reduction of a full boat tail. (Would you agree?) Do you need access through the tailgate?
Add a fold-down bench to sit on and you have a half truncation/half box cavity. In the 1930s the Oregon State government made the beaches public highways to prevent people putting fences right down to the surf. Which means you can tail-gate right on the sand.
If you want an interesting place that's very easy to miss, try
Tokatee Kloochmann State Natural Site. It has an elevated parking lot and view-point, but there's a cave underneath.