Biscuits?
 

Has anybody ever used biscuits to join the vertical and horizontal stabs? It just came to me. You could use a slot tool to start the opening, and just work it a little more.

Biscuits?
 

I've never used biscuits, but I have drilled a hole and placed a short piece of music wire in there.

Biscuits?
 

I use bisquits in my woodworking however they work on an expansion system. The wood pieces I join is thick enough and the grain strong enough so that when the white glue expands and seals the bisquit inside, everything is held tigthtly in place.

Not sure but would the balsa be able to handle the internal pressure of the bisquit pushing outwards? It's something you could experiment with but I'd advise you to try scrap balsa first.

Nuker