Well, I formed the frame out of 1 inch thick FoamulaR insulation. The ribs and rails were initially cut using a jig saw, then fine-tuned using a finger-cutting knife (actually, one of those As Seen On TV knives that are supposed to cut anything from a ripe tomato to an aluminum can, but the knife I used couldn't cut cooked meat worth a darn but was excellent at cutting fingers - hence "finger-cutting knife").
The height and width of each rib were calculated using Microsoft Excel, based off of
that paper written by that Naval PostGrad sailor, and my memories of how the snow formed itself on the tonneau cover of my truck last winter. I didn't want to bother with sharp edges, because they're harder to work with, with regard to being able to coat the exterior with fiberglass or something, so it was made as curvy as possible.
The ribs and rails were glued together with Gorilla Glue and a lot of water to ensure the foaming action necessary for the Gorilla Glue to work. The skin was made with 8 sections of FoamulaR 1/4 inch sheeting. Each sheet was partially cut, then partially glued onto the ribs, repeatedly until each sheet was completely glued onto the ribs. The sheeting was held in place by weights placed on the seams (things I had in the garage - spare crank pulleys, spare car radios, spare alternator, jumper cables, etc.) while the glue set.
The aerocap failed because the rear ribs and the side mounting rails were too thin to support the vibrational stresses imposed on the aerocap while the truck was travelling at speed. This was compounded by shrinkage of the rear 1/3d of the aerocap to the point where the rear was able to be wiggled by hand. The combination of the two causes caused the rear 1/3d ribs to fracture and separate from the mounting rails. The rear of the aerocap, no longer joined to the truck, caused the rest of the aerocap to break along the mounting rails, and then lift off the truck altogether. The aerocap flew upward about 10 feet, and luckily landed in the center divide of the highway, but the rear 1/3d completely disintegrated. I could recover the main part, but it was completely unrepairable.
I should have secured it more firmly at the rear, I should have used thicker ribs and rails, and I should have stiffened the skin with fiberglass. These will be applied to version 2.
I imagine that tuft testing would have revealed some turbulence around the rear of the aerocap, because I think the vertical slope was a bit too aggressive.
I was able to quantify a gain of 1 MPG from this aerocap. Incidentally, the same testing revealed absolutely no gain whatsoever from having a tonneau cover installed.