[Big Dave]

Much better. I wish I’d done mine this way.

Back on your question. You could slope your cap down from the centerline out but you can’t get too crazed about it. I did some three-dimensional trig on the problem.

You have established a ten degree slope on the centerline. This is good. The air flow should stick like Super-Glue. By my calculation, you could slope down away from the centerline at an eight degree angle (about a 1-in-8 slope) and maintain less than a twelve degree (1-in-5 slope) angle relative to the top of your cab, so the air flow should stay attached to the end. If your truck is 80 inches wide that would allow you to lower the height at the rail by 5 inches from the centerline.

Nice thing is it will shed rain like a duck.

If you read the thread about “commercially produced aero cap” you’ll see the evolution of Bondo’s cap. I can’t seem to find the video of it in the wind tunnel but I seem to recall seeing the smoke sticking to the cap all the way to the lip. That would make it the gold standard. Now Bondo held the height at the rails to the same as the centerline, but he is trying to maximize internal (payload) volume rather than seek the maximum wake area reduction.

Somebody (I believe it was Frank) posted some tips on heat-bending coroplast. This might be useful bending that Texas-sized political sign into a righteous cap.