From what I've seen,it takes a wind tunnel to figure out a proper geometry for a truck spoiler.
The following is a set of very degraded TV images I photographed on pause,from a VHS recording of a series of tests conducted by Texas Tech that I was given in 1990.
Every configuration has separation or overshoot except the deep,bulbous trailer nose at bottom right.
This is a tough DIY project with a limited budget.
