If you look at pictures of the exhaust dump diffusers from F1, the exhaust is routed to exit into the airflow so that accelerated the airflow going through diffuser. The exhaust didn't just dump straight into the diffuser at a right angle. This is one of those things that is far more complex than it looks.
Have you read the autobiography "How to Build a Car", by Adrian Newey? It's an excellent read, with his interesting story, but there are also numerous diagrams explaining some of the technical aspects of his car designs. Since he was the first to come up with that innovation, he discusses it a bit (chapter 73), and there is even a diagram to illustrate the concept.
One of the novel things they did was to work with Renault to come up with a system so that even when the throttle was closed, the engine still blew a high-velocity flow of air into the diffuser. They found that without this system, the diffuser efficiency wasn't very good. Again, more to it than meets the eye.
If you want to get some good ideas for diffusers, take a look at the current crop of IMSA GTD or GTLM cars. They have giant diffusers (with no exhaust dumping into them). They are quite large, extending past the rear bodywork a fair bit, and they flair outwards once the diffuser gets past the rear wheels. You would probably have to come up with a large apparatus that you will have to take off the car to load onto the trailer, sorry to say. Take a look at the diffuser on the GTLM 911, it's a pretty complicated piece of work!
I really admire your willingness to try new stuff! I'm guessing that just making the bottom of the car a relatively flat surface from front to the rear of the car will reduce lift somewhat, along with lessening drag.
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