1) These are good magnets to use for big, man-size plastics. I use these for lighter plastics. Metals and plastics using 40mm+ base inserts will want either the larger magnet or multiples of the small one.
2) Green Stuff
3) Super glue
4) Waxed paper
Mix up a ball of Green Stuff, about as big around as can snugly fit between the base and the surface it's sitting on, or a little bigger. Put a drop of super glue onto the bottom of the base, and drop the ball of putty onto the glue. Put another drop of glue onto the putty. Add the magnet; you don't need to press it way in, just a little bit so it catches in the glue and putty. Finally, turn the base over and rub it on the table, on a square of the waxed paper. Leave it on waxed paper to dry, to keep the glue from hazing your table.
I don't know if it matters much, but when I remember to do it, I score the inside of the base and the top of the magnet with a hobby knife, which presumably helps the glue adhere to the slick surfaces. In practice, the bases rarely come apart anyway.
It's a good idea to make sure your bases' polarity all goes the same way. If you ever want to stick a rare earth magnet to another rare earth magnet (e.g. I like to glue magnets to the tops of corks, so I can then stick a miniature to the top while painting it, and not have to use poster putty), you'll be happy you made them consistent.