WOW, this is a good one!
George Hicks did a whole thing of why a flat plate airfoil is best compromise for this area of flight IE very low RE#'s and wing loads for 3D flight not RC training, after he designed the Tribute, i forget where it was posted though, if you don't know George Hicks he is a aero engineer for Gulf-stream and an excellent pilot part of Team JR.
And as far as designing a better airfoil for these models, sure it could probably be done, but i bet it resembles a flat plate, like thin and blunt. I would love for somebody to compile a list of airfoils that harrier well for 46 size planes