Florian,
Like you, I didn't want to modify the planes themselves any. It is possible to mount both the ball bearing and the ring magnet onto pegs, & thus no modifications to the plane required. Only thing needed is a slight cut of the tip of one peg where it sticks through the ring magnet, & some super glue.
Since you'd have to cut the pegs, you might want a bag of them if you don't already have one. (They're a good thing to have anyway.). That's it!
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