General Tools & Instruments 376D 50-Pound Pull Shallow Pot Ceramic Magnets
Alnico magnets are strong and permanent.  They are made of aluminum nickel and cobalt and work under normal conditions and temperatures.  This shallow pot ceramic magnet is 3-1/4" in diameter, is 7/16" in depth, and has a 50 lb pull.  It has a 9/32" center hole for attachment.