There are 2 ways you can do this.
1. Get a piece of solid balsa the length, width and thickness you need and sand, and sand, and sand, and sand until you get the shape you want. (Actually not that hard, just an awful lot of balsa dust
)
2. Since the aileron is 1/2" thick at the L/E, make a bunch of rib "tips" 3/8" thick and tapering down. Then glue the tips to one piece of 1/16" balsa, put in some reenforcement for the control horn (either a solid piece of balsa or 1/4" lite ply depending on the type of horn you plan to use and how it attaches), then sheet the other side and add a L/E.
If you go this route, I suggest drawing a full size pattern and marking the location of each rib tip on the pattern (I suggest spacing the rib tips ABOUT 1 1/2" apart). Then measure and cut your "tips" accordingly since each tip will be a different length.
Method 2 is a little more complicated, but will definitely be lighter than a solid piece of balsa. Instead of tri stock for the L/E, I would use either 1/2 round balsa (special order ) or a rectangular piece of balsa and sand the shape you want/need.