Hertz is imposing a minimum length restriction from CVG for pickups on Sunday for whatever reason. If you make the rental at least 3 days and 1.5 hours you will see availability. You could do that and just return early and they should adjust the price accordingly, but no guarantees.

You could try renting from the FBO location but no guarantees they will allow you to pick up as you are supposed to fly in via private plane so YMMV.

The other option is to just rent from someone else. You can get a 1-day rental from Sixt starting at $50 all-in.