probably something like this or
this
The magnets were one of my favorite things I did in my extension holders. They aren't strong magnets, I went with something just strong enough to keep the holder from moving through a drawer liner but not strong enough to hold filings on top of the extension holder.
I chose the magnets I used for a couple of reasons.
I didn't need a lot of strength so while I dropped some epoxy on the backside of the magnets they are a press fit.
The open cup collects filings and while my design case is the placing them on drawer liners I don't want a magnet that is hard to clean that may be placed on a painted surface.
Not that other magnets wouldn't work but they weren't appropriate for my application.