"Rare Earth Magnets" Are what I've most commonly heard of people using. You can get search them on Ebay or many hobby shops carry them. As far as I am aware though, people fit them into place before foam filling the mask. I don't think they would be strong enough to work through the foam. Depends how thick it is I guess... But generally speaking I'm pretty sure it's done before the mask is filled.
Would it be possible to dig through some of the foam to get the magnets closer to the Latex? Or if the mask isn't painted yet, you could possibly cut through and slip them underneath. Then patch the mask up with some latex before painting it.
