Sounds like your assembling an ARF? When I build a plane I cut out the balsa area that the horns are going to mount in and add a ply section so I can use small wood screws. It's just adding hard points. There Are other ways but hard pointing is just a very easy way to do it. If it's an arf and the controls area is thick enough you can remove some of the balsa and inlay the hard wood without removing the covering from the top so it isn't seen.