KB: maybe you've arleady seen this.... but bondo's is the Swiss army knife of truck caps! It opens like a hatchback, can be converted into a conventional square camper cap with the addition of side & rear curtains, and the upper hatch can be removed entirely for transporting tall objects. See this thread for photos:
http://ecomodder.com/forum/showthrea...d-cap-583.html
Also, regarding flow effects, you may be interested to view on-road tuft testing of a tapered bed cap:
http://ecomodder.com/forum/showthrea...per-11522.html . That thread somewhat addresses the issue of the flow transition from the side to upper surfaces (in that case a sharp transition, more likely to trip flow, yet it still appears to work well - though the upper angle is shallower than most).