I have always wanted to try my mortising machine. It's used for square holes and slots for tenons in woodworking. There's a square chisel with a drill bit running through it. You work it sort of like a drill press; the bit removes the bulk of the wood while the chisel simultaneously squares the hole. I have a 1/4 bit & chisel for it but just haven't got around to trying it. I doubt it will work well on soft balsa but you'd be able to do a nice stack of ribs all at once if it did. If I remember right Rockler.com, Harbor Freight or someplace like that had a mortising attachment for drill presses at one time.