One example of undercar airflow treatments specifically called out by Hucho in Ch. 4 (I don't have the book in front of me, so I apologize for not citing the section number) is on the 1996 Opel Calibra. There's an illustration that shows the results of air dam testing, etc. and part (d) has a figure of the rear diffuser panel. It stops short of the rear bumper cover, with a pretty substantial gap between them, but the text states that the diffuser's effectiveness is still increased by removing the center section of the front airdam and allowing more airflow under the car--both of these struck me because they run counter to conventional aerodynamic wisdom. Things don't necessarily have to be perfectly smooth to get
some benefit; and, I think people tend to overestimate the "parachute effect" without any real information to back it up because we "feel" like it should be significant. It might not be, though--you'll have to test to find out.
I'm reminded also of the Audi A2 study from 2011; a completely smooth floor showed a 20-count drag reduction in a tunnel with no moving ground; that was reduced to just a 6-count reduction in C
D in a tunnel with moving ground and rolling wheels. The stock A2 has a fair amount of paneling underneath, but with a substantial central opening for the exhaust and openings around the wheel housings, all of which were covered to get that -0.006 C
D delta:
We're all really just shooting from the hip here. If you want to know if that eBay diffuser does something, the only way to find out for certain is to buy one and test it. Of course, then you'll run into the issue of noise in your results and how to reduce it so you can ascertain anything meaningful, which will likely be very difficult.