I've shot video of tuft tests on numerous cars by mounting a small camera on a boom that's attached aft of the area being filmed (obviously to minimize aero interference from the camera rig).
3 examples:
1) 2000 Honda Insight
From thread:
MetroMPG's Honda Insight boat tail extension (cardboard) tuft video; ABA test +9.7%
The cord tied between the two booms adds tension to prevent the camera from bouncing.
2) Similar set-up on my 2nd gen 2004 Toyota Prius:
From thread (starting around post #100):
The Premier's Limo: 2004 Toyota Prius "winter beater" ecomodding thread
3) And the current car (1990 Miata MX5) - I didn't get a shot of the camera boom itself, but in this case it was a rigid DIY tripod of aluminum tubing, liberally duct taped to the rear & side of the car:
From thread:
"MPGiata" modding thread: aiming for 50 MPG with a 1990 Miata. Update: success, 55 MPG
The other two tripod legs are out of the frame.
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Cameras with wider angle lenses are better because you can use a shorter boom (Prius boom is probably 1/2 the length of the Insight's).
Obviously this approach is only practical/safe if the road you test on is free of obstructions/pedestrians/cyclists!