Here’s one of my favorite clasps for polymer necklaces. It’s a rare earth magnet, hidden in a side bead. You open the clasp by sliding the bead open. It’s strong and because it’s not located at the back of the neck, it’s not constantly under tension and in danger of opening. Who said that necklace clasps had to be on the back of the neck anyway? Put them where they will work. They should be either part of the design or blend in.
for a great selection of rare earth magnets, try K&J Magnetics.