Good eyes! Yes that's a piece of 1/4 ply I inserted to catch the rear end of the dowels. It was only because I didn't have enough scrap dowel to reach all the way back to the main spar. It is not needed if you make the dowels longer (into the main spar ply doubler used to join the wing halves).

It's not hard and done with some scrap pieces. Mount the wing and drill through the wing and the ply attachment piece you epoxied at the rear with a bit smaller than 1/4". Remove the wing and drill it out to let the 1/4x20 nylon hold down bolts slip through and then tap the mounting plate. Note that the mounting plate is from 1/4" ply and the center has been cut away to let the aileron linkage work. A little bit of fast CA at all the drilled out openings (and the threads too) will strengthen all. Just run the tap through again after the CA sets up to clean the threads.


quint