As long as you have a good method for strengthening the wings(as others have recommended some very good ones here), a mid size or larger plane with a flat plate air foil could perform well as a casual cruiser or slow flyer. I would not use a flat plate wing on anything I intend to fly at high speeds, whatever size or scale(flat plate wings have sketchy pitch stability and some handling issues when they get up to higher speeds). The under cambered wing is probably the best compromise between weight, effort/difficulty, performance, and durability for most general types of slow flyer plane.
Some form of KF style air foil is an easy construction method for solving the strength and pitch issues of the flat plate, but I actually would not recommend it for a dedicated slow flyer based on my own experiences. I've found planes with this style of air foil to lose a bit of performance on the low speed end of the flight envelope, although they are very sturdy as well as stable in flight(including at high speeds).