A stock control horn could work, but only if the bottom hole in the horn is very close to the base.

The important thing to remember is that since the connecting rod between staggered wings is at an angle, the right angles needed for proper connections made at the top aileron and at the bottom aileron are going to decide where the connect points should be. And the connect point for the top aileron (on a top wing forward stagger) will have to be above the aileron just like the connection at the lower aileron. It will have to be as far above the top aileron as the connect point for the lower aileron is above the lower aileron.

The reason that I cut the base of the control horns and used only the "horn" part was because of the above. It was far easier to simply saw slots to epoxy those parts into the ailerons than it was to find a really short horn that would work for both the top and bottom ailerons.

If you do find a very short horn, then it can be located on the top of the top aileron and on the top of the bottom aileron. It will need to be overhanging the back of the top aileron of course.