I bought an ARF version of the LT40 many years ago, and it was a bolt on wing. I think its when Sig just started with the bolt on wings for the kadet.
All they had for bolting in on, was a little lip at the front of the wing to slide under the fuse former (you want to make a notch in the former so it fits nice and snug), then they just had (2) 1/2" blocks of basswood that were 4-40 tapped for the nylon wing bolt in the back, and 2 holes drilled through the wing. You may consider beefing that area up if its just balsa.

It may sound a bit confusing. Hope this helps. The hardest part will be making the lip up front, but make sure everything is lined up straight. Its not too critical on trainers, but you still want to make sure.

If you go this route, then I would suggest using T-nuts instead of the tapped 4-40 thread. I always used T-nuts instead of a tapped hole, but the tapped hole is still very strong if you put some CA in there.

Good Luck!