The Sun Below the Horizon Lights the Clouds

"After sunset the Sun is behind you, low over the flat plane - it can't shine UP onto the undersides of clouds that are in front of you, above the horizon."

Free - it takes patience. On a partly cloudy day, stay outside 30-60 minutes AFTER the Sun has fully set. Watch the clouds in front of you (over where the Sun went down) and look for clouds that are still lit, especially from below, and for 'sun rays' fanning out from a point on the horizon opposite the setting Sun.

High clouds keep glowing long after sunset - lit from BELOW, because the Sun is below your horizon but still above the tops of the clouds. The 'sun rays' (crepuscular rays) appear to radiate from a point on the opposite side of the sky, below the horizon.

The same effect has been photographed on Mars: Curiosity looked up after sunset and saw crepuscular rays from a Sun below the horizon - the same geometry, 225 million km away (PIA25739, the first Martian 'sun rays' ever imaged).

On a flat Earth the Sun is a disc always above the plane; after 'sunset' it sits between you and the distant sky and cannot light the underside of clouds above your horizon. On a globe it is trivial: the Sun is below your horizon, but the tops of the clouds are high enough to still be in the light.