I recommend Loctite Ultra Gel for any magnet of 1/4" or larger. (Cheapish stuff from Walmart).
For magnets of 3/16" of less, use Loctite Lizuid Long-Neck Superglue, (the cheapest stuff at Walmart).
The gel is great because it's got a rubberizing agent and is thicker, so it fills space better and is a bit stronger for larger magnets. The liquid is great because it fills the smaller spaces better and still holds up to magnets.
In general, try to score the magnets and clean all fingerprints off of the gluing surfaces before gluing them for maximum strength. A clean rough gluing surface gives maximum bonding strength.
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