Very good pics, Ed. Really outlines a most effective way of anchoring that end of tube....very similar to the "Arresti Fix" I had to do after Pawnee Rock last season on Arresti #1. The only thing I did slightly differently was to attach the "box" you fabricated to the chord-wise false ply rib. With lightening holes, it doesn't add but a few grams. I make the false rib full height from sheet to sheet, and usually at least 1/2 chord at that location.
Then, the fiberglass scrim that you show forward of the tube end box, I actually align OVER the wing tube area, and go about 2/3rds span. At the aileron cutout, I make a small rectangle of scrim to tie the hole to the "spar", if it interferes.

You have a very good system designed, looks like it would be easy to do!