Why not buy magnets something like this would work
My experience with these magnets is they are very brittle and it doesn't take much force on the surface to shatter them. In the case of the router mounting plate, the ring or coins are to protect the wood from the force exerted by the grub screws used to level the plate. Gravity holds the router plate and router in place, and depending on the design of the plate, additional countersunk screws or a side cam screw are used to secure the plate to the table.
In practice, the bottom of the plate does not touch the router table. The lip cut in the table top should be deeper than the thickness of the table. This allows room to raise the plate and level it to the table top.