[Big Dave]

I would think a half-height aeroshell, combined with a duckbill would give performance approximating a full (and properly sloped) aeroshell.

Obviously, nobody has done one, but there's always room for pioneers.

I'd start by using cardboard or paperand see how high you can bring up the bulkhead end of your semi-aeroshell without blocking off an unacceptable amount of your rear view. This figure would vary driver to driver. Tall dudes could probably tolerate the bottom 60% blocked but shorties might not hack with more than 30% of the rear view blocked off.

At say 50% you could slope your semi-aeroshell at nice 8 to 10 degree angles.

A wing or duckbill off the top of the cab would transition the flow down about 10% and the gap would be minimal for the flow.