After I started reading Andy Lennon's articles in Model Airplane News about 10 years ago, then later his book, "The Basics of Model Aircraft Design", I found this:

Flaps on symmetrical wings are pretty much a waste of time, although as wsmalley said, spoilerons may help. But you have a symmerical wing, and have the capability to GREATLY improve your takeoff and landing performance, with a little effort. If you like the plane, it's worth the effort.

Try slotted flaps, per Andy Lennon's articles. (or email me if you can't find them) Spanwise, the ratio should be about 60% flaps, 40% ailerons. Chordwise, the ailerons should be about 25% of the chord, and to avoid pitch coupling when you lower the flaps, they should be about 33% chord. It's not that much effort to add them, and they will give you performance that is much more scale-like for your Porter.

They're a much better solution than whatever you can do with strip ailerons and and mixing.