Excellent drawing!

As luck would have it, I found a piece of hardwood under the balsa about 1/4" from where the tube stops inside the outer wing panel. The cardboard tube sleeve goes through this hardwood block. So, using your idea, I will be installing a screw that goes from under the wing, through the block, through the tube, and then through the block on the other side, where it will fit snugly to stay in place during flight.

Should I be putting one in the smaller rear tube as well?