Add a layer of thin ply to the upright foam and I have a mount. The motor mounts are 31mm apart and the area is about 40mm x 40mm excluding the tabs. Add a couple of triangle support tabs behind the mount will add support.
In the background you can see the 19mm (0.75″) wedge that gives shape the the wing. I’m not sure if this wedge is supposed to be shaped or a straight line from the middle of the wing to the leading edge. So for version one, it’s a straight line from 19mm down to 0.
Before I start hot-gluing things together, the motor and servos need to be tacked in place to get the CoG right.