[ausias]

Best shape is what become a extension of your front car. Add a Spoiler to the roof (maybe 30 cm could be enough and begin the streamlined shape above drivers door. But if you could fill the gap between car and trailer hidden in front view with a kamback it will permit -35% reduction between car only S·cd and the new one. I'm designing and testing CFD with solidworks for a Van and even having a 110% front surface with a spoiler, the hidden boat tail trailer give me a total cd reduction from 0.36 to 0.233.
You need to fill the wake of your car and some 50 cm to 100 cm kamback to fill the wake of the trailer.

Some details of the images: 30m/s front wind, 108 kmh, 67mph,
Sfront vehicle = 2.5 m height, 1.9 m width, 4.8 m length
trailer 2.3 m x 1.7 m x 4.3 m
resulting length 9.6 meters :-O
drag force reduction -35%, expected fuel consumption saving -25% to -15 % L/100km (+33% mpg to 17% mpg) unloaded(2200 kg) to 3500 kg total mass.