I saw these retractable bed covers some years ago and had forgot about them. Now what if you changed them just a bit and mounted them across the top of your cab with solid sides. If you want to put something tall in your truck bed, retract the cover and use the bed as originally intended. when not open, the cover provides the 11 degree slope for aerodynamics.
I think a rolltop aerocap is an idea worth at least designing. Some practical challenges that come to mind include rounded corners, structural integrity and weather proofing it especially for freezing climes. Trucks have a much narrower roof than bed so a slider made for longbed stepside could provide donor materials for the project although cost and availability seem iffy. Fun challenge!
GM got a patent on a 1/2-tonneau cover which had lower drag than a full tonneau.
The back half is covered.The front half is open.
So aerodynamically,the retractable feature would need to be reversed.
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