Def earth magnets are the way to go. If your mask is foam filled, just cut two deep holes (I just used my finger) and put two magnets CLOSE but not right against the latex and about 4 inches apart. Those suckers are strong and if you have the magnet right against the latex, it could tear the latex eventually.
Then just two more magnets glued with epoxy to the inside of the Bio. The hard part is lining them up properly.
I'd also recommend using black felt and gluing it to the inside of the Bio to avoid any paint getting rubbed off of your mask.