Picked up a new router bit. A 1/4" diameter, spiral, upcut bit. This allowed me to cut a slot for the narrow end of the wing attach plate and still get the full 2" depth plunge that I needed.

I practiced twice on a couple of laminations of 2x4s that I had screwed together. This allowed me to practice the handling of the router with this long bit and it allowed me to experiment and practice with the aluminum angle sections as fences. Firstly the 2x4s were screwed together well outside the area that I was going to route and I made sure the screwheads were well below the surface of the 2x4. I then levelled the surface of the 2x4 lamination with the thickness planer. Clamping the table together like I had done previously and then clamping the fences to provide guidance for a nice straight 8mm wide slot the full length of the attachment plate.
I experimented with the depth of the router bit for each pass and after a few successive passes, I was able to get the full 2" depth needed to bury the attachment plate.
After the 8mm wide slot was cut, I moved the fences outwards 2mm on each side and cut the 12 mm wide slot. Repeated twice more to get all 4 widths cut and the attachment plate to fit.
After 2 hours experimenting and practicing on the cheap 2x4s, it was time to tackle the lower spar cap. Fortunately all the lessons were learnt with the 2x4s and cutting the notch in the actual spar was a non-event.