Here is the plan,

The lite ply is cut and fits in like a glove, from bulkhead to bulkhead and fuse side from fuse side. The 1/8" slots lock the 1/8"ply then 1/4" will be added to get flush on the wing saddle. for a total of 3/8" wing mount. I think the idea of adding a pin to the 1/4" would do the trick. It is a method I use on some of my scratch builds, and it makes a strong uni body structure.
ED