[Christ]

You can make a cleaner "edge" where your kamm joins to the sides by using wire and silicone or epoxy. Put the stuff in a dispenser tube, goop a bunch of it into the holes, and stuff the wire into the goop on both panels. You need a way to hold the panels together for some time, but once it sets, just duct tape the seam and you're good to go.

Another thing you can do is trim back the lower layer (and the corrugation) of the side panels, allowing like a 1" overhang of just the upper sheet of the coroplast, fold that over the top sheet of the Kamm, and rivet liberally. Then duct tape, of course. This is easier than the first method, and should be just as stiff.

A combination of the two will work just as well, if not better.

If you want to take the whole thing back off the car temporarily, you can even stitch it together with fishing line in a large sewing machine (or by hand, if you're a masochist).

Cut/melt the corrugation out of the side sheets back about an inch, let the top and bottom sheets of the side panels overlap the top panel as it does now, and get to sewing!