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What happens if you need to carry a load in your truck, which means the tail end of the cap is up in the air by 2-3'? ... Last thing that would help would be a giant air brake! 
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Which is exactly what you would have. Nothing worth doing is easy.
Alligatoring open is only one possibility. You could have triangular sail panels with 3 or 4 retractable sections that stack up under the front. You could have 3 or 4 hoops that fan out from the middle of the bed sides with a heavy canvas top like an old touring car.
Maybe a straight cap with a 45° chamfer a la Morelli? But then you're still limited in load height. Work out the use case. What would it be OK to not be able to do?