I used
these. The 250 pack I needed for 4 x 61 note manuals cost around £40.
They keys are centre pivoted, 240mm from their front edge and I superglued the magnets underneath the keys 90mm behind the pivot point. A 20mm long M5 bolt passes vertically through the key above the magnet for the magnet to pull on, and it is adjusted so that there is the tiniest possible air gap between its end and the magnet (avoiding a loud click as the key returns to position).
A weight of 150grammes applied at the key is required to overcome the top resistance, which then falls to around 100grammes to complete the key movement and hold it down, which is pretty close to what I've found on the internet as recommended values.
Err on the side of having too strong a magnet, because you can adjust the bolt away from it if it's too strong.
According to the magnet website, the attractive force between a magnet and a piece of steel is the same as that between two magnets.
Caveat, I've not yet completed the console so can't vouch for how it feels when used in anger, but on the few notes I've set up and played with (and measured) it feels pretty realistic.