Use wood screws into holes in to the plywood horn plate on the underside of the aileron.

Drill the holes 1/16", thread in the screws, take them out and harden the new threads with thin CA. Allow it to cure. Put the screws back.

BTW, why are you using the large horns in the first place? These are really meant for planes a lot bigger than this one.

I thought the Spacewalker came with horns. They're OK to use.