[aerohead]

*diffuser angles as steep as 11-degrees will reduce drag.The lowest drag is achieved with 2.8-degrees and a long diffuser.
Second best is a 4-degrees on a 'short' unit.
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*Those two canopies should work well for you.
If the roll bar must be so tall,let the tubes penetrate the canopy top to allow proper streamlining of the canopy itself,then streamline the roll bar hoop.You can use airfoil-shaped foam rubber,taped onto the tubing.
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*Nose: yes,the delta-Cd -0.14% nose is perfect for you!
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*'Fish',yes,as Freebeard has already mentioned, fish ,birds,traditional airfoils,and streamline bodies will have their thickest section within their first 1/3rd of length.This allows 2X the distance to decelerate the flow back to unity,with zero separation and full pressure recovery.
Speed Demon evolved from the Ack Attack car which was inspired by a Coho Salmon fish.Burt Munro's Indian motorcycle streamliner is very much like a trout fish
If you cannot do this taper,then just do straight sides,like many of the submarines,back to the point where you can begin the boat-tailing.