David, I referenced the 1/2" square maple beams as material that I used to make a mock-up joint. In the actual SV-11 ARF/ARC kit, they supply two ~6" long spar joiners. I'm not sure what the wood species is - it's of Chinese origin - but it's strong enough for what we need. I simply cut the 6" pieces in half, so about 3" gets glued into its respective plywood box, which is in turn tied to what looked like a 1/16"x 3/8" (? eyeballing here) spruce (? or something like it) spar that runs the length of the wing. Running maple beams the entire length of the wing would add gobs of weight and is not required for the sufficient strength.
Note that most foam wing builders run lite ply spars only about 1/3 of the span - that's because that's where most of the loads are concentrated.
If you are building from scratch, I would recommend starting with 1/2" square maple beam stock, cut about 3" length, taper to about 1/2" x 1/8", and attach to a spruce spar (either 1/4" square, or 3/8" x 1/8") by wrapping with a strong nylon thread (Jo-Ann fabrics) or, better, Kevlar thread, and soaking the thread with thin CA. Then proceed building the wing halves as usual - ribs, sheeting, etc.
And yes, this will definitely work on built-up fuse as well. That's how all the Ukrainian take-apart planes are built (much fancier CNC hardware, but the same idea).