Oops, almost forgot.

For a slower flying model put in the extra effort to cover and build with an undercambered airfoil. The one on my Flamingo has a Gottingen 501 and it works great. Super slow when wanted but a surprising turn of speed with the nose trimmed down a touch to punch out against the wind. It's no racer of course but it does surprisingly well for what it is. The higher mean line camber of the "undercambered" shapes will provide a lower stall speed than the other options. It's like having lifting flaps deployed all the time.

If you insist on flat bottomed then there's really only one sensible option. The basic good ol' Clark Y. There's more modern flat bottomed sections used by some of the sailplanes but they are lower camber than the Clark Y and set up more for speed than slow flying as you've described.