the strength of a magnet is one thing, but when it comes to magnetism, size definitely matters. the strength of the earths magnetism is about a half a gauss, but what a magnet!
get you some old big strong speaker magnets, make sure the polarization, and set your container on top of the magnet with the proper pole facing up. if you want really good magnets, kill a microwave. its the only merciful thing to do.