BMathews, thanks for the reply. The fins are built very light and would probably break off without damage to the anchor point should the plane flip over even if they are glued in place. That is an issue I had not thought of. Digging out the remains from the slot to repair/replace a broken fin would be ugly.
I agree that the in-flight forces may not require a hard anchor but lost of a fin in flight may not be good for flight characteristics (unstable yaw). I would be nervous that the fin would pop out if it was just held in by friction. They currently fit pretty tight but that will loosen as they are reinserted repeatedly.
Maybe a threaded rod embedded in the fin with a nut on the bottom of the wing will suffice.