I used to make 2m wooden F3A planes-at least the fus sidesthe shape was made up with foam/veneer turtle decks top and bottomwith longerons at the "box" corners.
Getting the balsa sides deep enough is no problem-just butt glue the sheets along their length, much as you would for wing skins.
To get the length,I used to overlap two lengths of balsa so that the joint would be approx 45 degrees, fix the overlap with double-sided tape, and then cut a deliberately wavy line through both sheets with a powered fretsaw-this ensures that the joint can only go together in one position, when the parts are separated, and subsequently glued.
The only hi-tech material I used(in the fus sides) was 60 gsm kevlar cloth epoxy-bonded under clamp pressure, to the balsa sides-before the final shapingthis allowed the use of 6-8lb 1/16 balsa-together with 3/8 square, or 1/2 triangular, balsa longerons,this was plenty strong enough.