ORIGINAL: Test005
Thanks, I think I get the idea.
You mean to use the spars as 'cieling & floor' for the box and then add webbing on both sides to make walls to create a rectangular hole in the wing to hold a joiner?
Yes, that's correct. Use 1/16" or 3/32" aircraft ply for the shear webs and join with epoxy. This is not a place for CA.
Alternatively, you could use a round tube joiner with sleeves in each wing half. Just cut slightly oversized holes in each rib (I would go out about 3-4 ribs on this wing) and then glue close fitting ply donuts to each rib to fix the sleeve in place. You can buy tube and sleeve sets at a number of places and it is a pretty straightforward modification which will give years of trouble free service.
Mark