Maybe my little test with the sunset hasn't convinced you, although as I write this, no one has come up with even an attempt at a flat-Earth explanation for the results. But here's another test, which requires quite a bit more time and effort than the last, but directly indicates that standing water curves, and even gives you a way to approximate by how much.
It is a variation on the test performed by biologists Alfred Russell Wallace at the Old Bedford Level in 1870. You can read more about that event here. But it requires a long stretch of relatively still water with a line of sight over at least five miles. And if the water is not shallow, the process can be rather harrowing.
So, I'm calling on flat-Earthers who live in very cold climates to try a simpler variation. You'll need a sufficiently large body of water that freezes over so that it's perfectly safe to walk on. You'll also need three stepladders that are at least 10 feet high (Gorilla or Little Giant ladders would work well), two targets big enough to see from miles away with a telescope, some hardware to affix the targets to the top of the ladders, and a telescope. A spotting scope would be easier to use than the type designed for astronomy.
Set up the ladders so that one is about 2-1/2 to 3 miles away from the first. Affix a target to that ladder and measure its distance from the frozen surface. Set up another ladder so that it forms a line of sight with the other two, and is the same distance from the second as the second is from the first. Affix a target to it at the same height.
Now go back to the first ladder and set up the scope so that it is also the same height above the surface as the two targets. Aim the scope at the furthest target. If the Earth is flat, the first target will be in the way of the second. In the Earth is concave, as "Lord" Steven Christ claims, the first target will be below the second target. And if the Earth is convex (and it is), the first target will be higher than the second.
If you measure how much higher, by having someone with a cell phone move another target until it is centered in your scope, you can use that information to estimate to circumference of the Earth, not with any great accuracy, but close enough to know that the "official" circumference is definitely in the ball park.
Have I done this experiment? No. First, the sunset tells me what I need to know, and I don't feel the need to go to the trouble. Second, Wallace did this in 1870 and I have no reason to doubt his veracity. And third, if I did the experiment and published the results, the flat-Earthers would only reward my efforts by declaring them fake. See, for example, what happened to Wallace.
That said, when the lakes in my area freeze over this winter, I may suggest to one of the science teachers at the local high school that this would make an interesting project to show that direct evidence is available to anyone willing to access it and evaluate it with an unprejudiced eye.
The question is, are any flat-Earthers willing to go out into the cold and do the same?