For this you will need
sells very strong small rare earth magnets for low prices, I bought 24 half inch magnets for $8 + $5 S/H Product NSN0640, they have a 2.5Lb pull, totaling 5Lbs. when you put two together. Id recommend something in that range as weaker magnets, that you might get at a supermarket or the like, might not have enough pull to do much. Speaking from personal experience, I threw away one shower curtain with a strip of cheap magnets that hardly kept the shower closed.
$1 from dollar store
$1dollar store again
$1 from dollar store. You might be able to tell I live down the block from Jack's, the biggest dollar store in Manhattan