Hi Midair,
It was fun combatting w/ you last week. I use Pro-Bond Poly Glue on my foam wings. I put only a very, very small bead of it in the spar slot, then I put the fiberglass rod in the slot, and then put a much heavier bead of glue on top of the rod. Right away after this, I put a strip of Bi-Di tape over it. This way, when the Pro-Bond expands, it pushes the spar down farther into the slot, and the bi-di tape ensures that the Pro-Bond doesn't expand up (at least not enough to hurt anything). Works good for me, and once the Bi-Di tape is on top, it's done, no need to worry about the f/g rod shifting or popping out of the slot! Take care.