I'm looking at
this set of plans for the Victory -- that's not an I-beam wing in the peculiar sense of the term we use in Stunt. The flight loads are being carried almost entirely on the leading edge sheeting, with the join in the center reinforced by the center section sheeting.
You
might be able to pull it off with a plywood tongue and socket (sailplane style) right in the wing center section at the spar, with both tongue and socket carried out to ribs where the center section sheeting ends. Carbon fiber might be better for the tongue -- a 3/32" or even 1/16" plate filled full of lightening holes would probably be more than enough -- you can keep the resulting dulled out drill bits as trophies of your effort.
This is going to completely change the stresses on the wing in the center section -- basically, instead of the wing wanting to snap in two right at the edge of the fuselage in maneuvers, it's going to want to fail either at the center, or much more likely by snapping right at the end of the tongue. So you'll want to figure out how to reinforce the spar at that point without adding too much weight.
On the plus side, the center section sheeting won't really be a structural member any more, so you can safely use 1/16" balsa there.