Found some time today to work on the "double chassis" idea to get both: a constant working airflow underneath while still having a working suspension.
I worked out a simple solution to attach the underfloor to uncouple it from the suspension movement by putting small screws into the underside of the suspension arms near the tires. This easy and fast idea takes out around 60-70% of movement, but less than I expected. Another problem is that I have an arched rear part of the floor because of the diffuser (and how I built it). So the front effect was pretty good, but the rear effect was poor.
Now what
I guess I have to rebuild the whole floor, need to buy a heat gun and use some other tools to make a perfect flat floor. The next step would be to utilize the (steering) hubs to attach the floor to. This way should lead to a satisfying airflow and chassis movement. May the force be with me
Edit: I did some changes to the floor today to try out the double chassis configuration. The attachment to the hubs is a challenge. The available space is hardly enough. Now all works out with around half a millimeter of space between wheel and hub.
This is how it looks right now. Next week I will start building an optimized floor with less cut out area for the front and a more nice geometry.
Got my excitement back. I wonder how the car will perform outdoor at high speeds with a sealed aero channel. Let the dust fly