I just made the aileron stop using the angle based on the plan. It didn't take that much effort or as difficult as I had feared.
To mark the location of the stop, I used a strong magnet, salvaged from a used hard drive, place a small steel plate at approx the location where the aileron stop will be, move the aileron to about 30 degrees up and the bracket will move the steel plate to exactly the location and angle of where the stop would be positioned. The magnet holds the steel plate in place as you remove the aileron. Mark the reference line with a sharpie pen and this is where the stop will be positioned. Make the stop using the scrap longeron angle and rivet. It will be permanent instead of worrying about changing the plastic donut every few years.