There are earlier threads here in Orchid on the soldering of magnets.
We successfully solder gold plated magnets, while respecting that
magnets will be disabled permanently if overheated.
You could consider mechanical retention of the magnet down in the
tube, by a circlip or wire clip, inserted in an internal groove just
above the magnet.
And, given suitably designed jewelry, there are low-melt-temp bismuth
metal alloys that are designed to shrink on solidification. You can
grip magnets that way, without worrying about the
solubility/life/reactivity of glues. If you use a low-melt-temp
alloy, be wary of many that include lead and cadmium in the mix.
Probably easiest of all, there are small ring magnets 5mm OD x 2mm ID
x 2mm thick which you can find on auction sites. These could be
located over a central plug that you gently swage over to retain the