[aerohead]

In past times, cars have been tested on top of flatbed railroad cars, pushed from behind by the locomotive, so as not to upset the airflow coming at the car.
Drag force measuring equipment was below the 'floor' of the flatbed, leaving only the vehicle exposed to the flow.
If we take a 30% test section blockage-ratio as the absolute maximum allowable, for zero-yaw flow conditions, a trailer which surrounded a car, with an open throat large enough to respect the 30% factor, would be so enormous that one couldn't use it on public roadways.
Also, rigging up test equipment sensitive enough to measure forces would, be in the way of the airstream ( a sting ), and road vibration conditions might knock the equipment out of calibration before you could capture any useful data.
If you had a hill top, with very strong reliable near-constant velocity winds, you could build a tunnel with a tail-vane, large enough, on top of a circular rail track-lazy-susan, which would slew it's inlet into the wind at all times. Measuring equipment would be under the floor.