You might just want to add a set of split flaps to the bottom of the wing. You wouldn't even need to cut into the wing structure. Just take some 1/16" plywood and lay them along the buttom skin of the wing, hinged at the front. You don't have to go to a bunch of trouble to fit them into the outline of the wing. THis little thickness won't make a difference to the way the plane flies.

You could even hinge them with hinge tape at first, though a more secure method would be better over time. Just make sure the flap addition doesn't interfere with down aileron motion.

For a split flap diagram, take a look at : http://142.26.194.131/aerodynamics1/...ge5.html#plain

Scroll down a little more than half way and click on the "Extend Split Flap" button. That'll show you what I'm talking about.