nightflyer,
I have built a model of the Cessna 120 and had the same problem...how to build the wing carry thru so that I can make a scale cabin. I was building from a plan so I did as it was drawn but I think I'd try to do what was done in the real thing. The main load on the spar carry thru is compression so I suggest using brass (or aluminum) U channel or square section to tie into the wing spars . If the wings need to be removable you could use pins to attach them to the fuselage. Metal is good in compression so a channel or square tube section of fairly small size would work and not interfere with the scale interior. Of course the carry thru parts would have to be securely attached to the cabin structure. In order for this to work the struts need to be functional and a solid mount to the bottom of the fuse is necessary.
Here is a sketch of the idea.
Good luck with your project.
Paul