Back in 2003 I was on a flight from Phoenix to Atlanta. While over Texas at 39,000 ft, with a pretty clear airmass containing a patchy layer of cirrus somewhat below our flight level, I noted some interesting phenomena.
1) The curvature of the Earth's limb was actually detectible. It definitely was not linear. This was not due to window distortion.
2) The cirrus cloud out at considerable distance, even though at a *lower* altitude than the plane, presented its *underside* to view as it was seen to curve downward over the curving planet.
3) A straight edge oriented to aim out one side of the plane directly in line with the horizon was very clearly seen to be aimed above the horizon seen out through the other side. I repeated this numerous times, so as to confirm the observation did not result from some momentary tilt while swivelling my head to sight along opposite directions. Again, window distortion definitely could not be so egregious as to mimic this. This revealed the horizon dip, whose angle at 39,000 feet is nearly 3.5 degrees. Which supports the perceptible limb curvature.
The sighting of the distant cirrus deck's underside from a higher altitude would be a fairly common sighting for the observant flier. It's not particularly obvious due to the generally diffuse form of such ice crystal cloud. My experience as a weather observer for Environment Canada helped to better interpret the form and illumination, but any reasonably observant person can discern the geometry when the conditions are favourable.
It's quite striking to see the cloud layer top side when nearby and out to 'intermediate' distance, thereafter transitioning to and 'edge-on' aspect and thence to a from-below aspect at great distance. When the airmass is nicely clear, the cloud deck's continuing arcing to a more downward angle as the planet's limb is approached is fascinating to witness.
If the flattards go primarily by what the can see and interpret directly, they should spend time gazing out airliner windows, and bring along a straightedge.
Actually, it just now occurs to me that a simultaneous sighting along opposite ends of a straightedge is facilitated by a small mirror affixed to one end. It is oriented perpendicular to the straightedge so that the reflected image of the edge is presented as an unbroken continuation of that directly seen. To confirm that window refraction/distortion is not the cause of the horizon depression at altitude, observe also at as low an altitude as is feasible--even on the ground while taxying.