If I had to do the fitting of the ailerons all over again this is roughly what I would do. First thing I would do is sand the leading edges of the ailerons themselves about 2mm larger than final size. This is going to be your final clearance when you sand it off so it is very necessary. If you look at these pics you will see that ultimately, the aileron leading edges get sanded right down to the rib noses - so add two millimeters to this, then proceed.
Next, sand concave one surface of the triangle stock. I made a special sanding dowl and sanded against a board (which happened to be my sanding block):
The plans show the triangle stock attaching to the back of the plywood piece on the wing (I think it's F16) and the thin balsa cover over it. That did not seem like enough surface to me so I added some light 1/4 square sticks between the ribs to also glue to. Quite possibly unnecessary but the additional weight is insignificant:
I clipped on a piece of liteply to the back of the wing to show the angle of the back face of the parts we're discussing. This is critical: the face of these parts MUST be 90 degrees to the surface of the wing ribs as they taper toward the rear. If they are not 90 degrees then when you glue on the triangles, the top surface of the triangle is not going to match the adjoining wing sheeting. I guess it could still be sanded to match but this part is going to make all the difference in how the wing/aileron interface looks so start with if very close if you can. ALSO: you will need to make sure when you glue the triangle stock to the interface that at least a little of it stands higher than the wing sheeting (of course). I found that fitting the ailerons took significantly more time than actually building the wing itself!
Here I've clipped a board to the aft face
Here's what I saw: the angle it too acute. See how the top aft surface of the wing rib slopes down? We want the angle of the top of the triangle stock, when glue on, to follow this slope so it matches the wing sheeting. The aft face of the wing openings must be sanded to accommodate.
Then you will glue the triangle stock on, finish sand the clearance into the ailerons and sand the triangles to match the wings. Good luck. . . . . . . .
Here's what it looks like when done:
Whew! I'm glad that's over with.