Russell, are you after balsa for a specific reason? IMHO, the age of the balsa trainer is over. Rigidity is just about the only advantage that balsa still holds over the modern foamie. The consequent precision is great for IMAC pattern and advanced 3D, but for a trainer that rigidity is a disadvantage.
I'm certainly not as experienced as some of our club members, but to my way of thinking the ideal trainer is robust in the sense that it can absorb damage, wide in its flight envelope so that it can teach orientation, and versatile to the point where it can be flown in just about any open space.
Orientation is the tricky part. A trainer that can teach orientation is an enabler, because it opens the doors toward other planes with better performance and looks. For those who feel they're sufficiently hooked to invest that kind of effort, my suggestion for the ideal advanced trainer (after the very first aircraft whose only purpose is to be as gentle as possible) is an EPP profile foamie, a.k.a. a shock flyer.
Take it to the park in the morning, during lunch, or after work - or all of the above. Get a parallel charging board (paraboard) and half a dozen packs for it. Fly it right side up, upside down, knife edge, away from you, towards you, upside down and towards you, yaw it sideways like a car... Why not? It's practically indestructible, and if it does tear during a hard smack it's only a matter of hitting it with some CA or Uhu Por (better) and it's back in the air.
I sincerely believe a month of that will teach much more orientation than a year of weeked circuit flying with some balsa monster which requires half a kilometer of runway and 6S packs. Nothing wrong with either of those approaches, but for those who want to train fast(er) the key is simply practice. It's possible to put fifty packs through a profile foamie in a good week, even with one of those pesky full-time job things. Not so easy with a traditional balsa trainer, especially after the first time it goes "crunch" during a hard landing.
It doesn't really matter which profile foamie either, though some are better than others. I've never flown one of these, but I have experience with the components and they seem entirely spot-on for the size:
http://www.hobbyking.com/hobbyking/store...ouse_.html
OK, so it's butt ugly and some of the scale fans will laugh, but with $3 servos and those recommended 2S 500mAh Zippy compacts for $4 each, it can teach someone to harrier and hover in their tiny back yard (my personal predicament) for like sixty bucks in the air, and by the time it's carrying its own starting weight in glue the pilot will no longer need a trainer
Just another option to consider. It's all fun and good in this hobby