Ack! Sorry about that--We flood those little sockets with superglue when we attach the magnets--but perhaps that nickel coating on the magnets is not porous enough to accept the bond...
Good idea on the screws though...I'd added those holes to the pockets in the bracket to give the air somewhere to go when the magnets were pressed in--but they could also be used screws as well.
www.mcmaster.com is where I go when I need a magnet right away--but if you have a week to spare-www.magnet4less.com sells them for like 1/10 the price! I'll have to check for a countersunk magnet that is 1/4x3/8 there.