For a person with eye level at 6 feet, the horizon would be 0.0419° below eye level.
To do an experiment to test this, we need a way to measure very small angles. A very cheap and easy way is to use an ordinary ruler.
Let’s take a 30 cm ruler, place it at 57.296 m and voilà! The value in meter gives almost exactly the same value of angle in degrees within maybe 0.1% error (guesstimate). I.e. the 0.0419° value should be at 0.0419 m.
Next, we need to have a very precise level. An easy way to do this is to have a hose a bit longer than 57m. The hose should start at the reference point and end at the observation point. Both ends will serve as an horizontal reference.
The water level should be at the 0 of the ruler and a telescope should be at the height of water at the other end.
While you can rent a theodolite, not everybody is familiar with this and its principle of operation may not be recognized by flat earthers who will say most probably that it’s a free mason instrument.
This experiment is very easy in principle and works with easy to understand concepts, and it shouldn’t cost an arm and a leg. It can even be implemented in a permanent installation. Happy experimenting!!!