I haven't built a 'plank' in many years, however I will make a few observations.

The airfoil would look more like your picture #4 with a reflexed TE. This is what provides the downforce at the rear of the wing to counterbalance the CG up front pulling the LE down.

It should be fairly thick to allow a wider range of angles of attack. Thin airfoils were just too twitchy. CG get to be pretty critical with thinner airfoils.
If you have Profili (http://www.profili2.com/), look at the Eppler series (they have a couple of reflex airfoils that would work), and also look at the 'Horton Standard', 'CJ4', and 'RAF 34' airfoils in the database. These are all sort of what you want.

You could also do a Google search for "Flying Plank" and find lots of info on full sized planks and airfoils for them...

Happy experimenting!