I've seen people use pictures of lakes to claim the earth is flat. Your standard household mirror has more error than a lake matching the curve of the Earth, but I'll bet you've never measured that or noticed any distortion. Unfortunately it's not possible to find completely flat water anywhere in the world big enough to distort the reflection because of curvature. Do the math and get back to me, I'd love to see it.
Someone needs to create a mirror with the same curvature as the earth and mock up a model. In these pictures the area of water shown is just a couple hundred meters wide in many cases, but in some it's a few miles. Design a mirror that has 0.03% convexity (which is 3 miles of curvature) and reflect a landscape.
"A normal household-mirror made with float glass may have flatness tolerances as low as 9–14λ per inch (25.4 mm), equating to a deviation of 5600 through 8800 nanometers from perfect flatness."
I'm going to run with 5600nm/inch, the low figure
3 miles is 190,080 inches 5600nm/inch of error 1064mm of error across 3 miles So call it an even meter Earth curve is 72 inches in 3 miles, so about double a household mirror