The structure consists of a hedgehog body made of 9 mm thick beech plywood.
I drilled 7,5-8 mm and approx.
4,5-5 mm deep holes into the back of the hedgehog.
Then I pressed three disc magnets S-08-05-N
into the holes.
The magnets stay in place even without adhesive.
In the end, I "painted" them with a soldering iron.
It is important that the magnets don't get hotter than 80° C, otherwise, they'll lose their magnetisation.