The hinge line will be around the 60% point of the chord...the picture will illustrate what I'm getting at although the structure is far from finished. The uncovered balsa sheet will be the flap surface and will pivot at the top edge some predetermined angle.

That was my concern with Monokote hinging...I'm working on a way to either put pinned hinges or CA hinges. CA is probably the easiest, although I haven't use a CA hinge in years (don't generally like 'em). I'm not sure I follow on slotting the flap...split flaps create lift and a ton of drag and aren't supposed to be slotted.

I'm planning on using fairly strong mini servos (thinking Hitec HS-225BB...67in-oz of torque) and I am using a 6V power source. I'm going to set up the linkage so the arm and link are colinear when the flaps are down...if the servo has the grunt to get the flap in position, then it can hold it with very little effort.