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I was planning on setting my circular saw to 1/8 inch deep and making kerfs in the back in the direction (or two) that I wanted the wood to bend. I know it isn't the best way to do it but I don't think it will affect the strength that much if I fasten it well. This is just a "proof of concept" cap anyway. I'll be happy to use it until June. So epoxy coating is not really a priority. If it significantly improves my mileage on the road, I'll make a more robust version with some kind of hatch back door thing and windows.
I'm fairly convinced that this is a good direction to start with by tapering in the sides and having a curved arch top profile. Most folks don't taper the sides and I think really only end up improving the airflow over 2 or 3 feet down the centerline of the truck bed. By taking the time to do this, I'm managing all the air above the bed which is about 8 linear feet of the slipstream (22" tall sides + 52" wide roof). After reading a bunch of technical papers today over in the "General Efficiency" forum, I'm getting a little concerned about lift forces that might be generated at highway speeds (with a head wind, passing a truck and 82% humidity). I don't think it will significantly affect handling, I just worry that the cap will come loose and take a short flight. I'm going to double the hold down system I was originally planning to avoid having to fill out a EAA membership, although I'm certain Burt Rutan would love having me in the ranks.
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