When following the instructions to build a RGB Minecraft Ore Lamp, I couldn't find a pickaxe with a slot for the magnet; therefore, I created one.
At 100%, the pickaxe is 65x65x5mm and the slot fits, a 10x10x5mm magnet.
I also included a PDF file if you want to glue it to the top so it can look even more with the "real" one and also to hide the magnet.