Remembering that "scrap" is Swabian for "stuff you just haven't found a use for yet," I locked the aileron in neutral position with some tubing and zip ties.
The idiot proofing really helped out, as this determined which control horn was put on the end of the aileron spar:
In this case, since it's the left (and upside down), the horn is also upside down (reinforcing curve thingie on bottom).
The horn is put level with the compression strut using the roll of aluminum tape and a bit of plywood.
Drill and bolt - since it's upside down, the nut is on the top.
That done, I turned my attention back to the ailerons. I don't like how there's these great big holes where the hinges are, as they're big drag catchers with the air driving right into the aileron itself.
So I came up with a bit of aluminum that I quickly bent to form a box around it.
I made one for both, and both are narrower than the leading edges so they can bend with the fabric. I'll rivet and seal with aluminum tape tomorrow, which should do the trick.