Hi Mike, I'd say that cyanoacrylate is good, but it has some perils and is, to a certain extent, hard to control. It might be the strongest, though. Another possibility would be to use a tiny piece (1/2 to 1/3 of a match head or a 2-3mm ball) of Sugru moldable adhesive on the back of the magnet.
* Then tap the magnet into place using a plastic, aluminum, or brass rod (anything non-magnetic so it doesn't pull on it when you try to withdraw it).
* The Sugru will fill any voids and will bond to the magnet. Let it set for 24-48 hours. (it is a slow-set silicone adhesive)
* You can try the same thing with other epoxies, but you risk stuff wiping off on the sides of the hex channel. Sugru won't do this. Whatever you use has to be very thick.
* Be prepared to clean the face of the magnet if the Sugru squeezes out.