Looks much like the setup Byron used in the day. Assuming the spars are going into a foam wing, you want a ply box structure to capsulate the spars and spread the load through the foam wing. This box structure will be epoxied into the wing and the spars glued into the boxes. The spar receptacles will need to be bolted to corresponding internal plywood bulkheads that line up with the wing spars. The screws in the bag thread into the spar receptacles and tighten against the actual spar thereby trapping the spar.