I like the bead idea and I am sure it works great, but would it not make more sense to put the bead on the edge of the lid, instead of the inside edge of the lip and then have the gasket on the inside edge of the lip on the deck. If it as watertight as you say it is, would it not help to keep water from getting near the magnets? I know, the magnets are sealed well under glass, etc. but this extra precaution might help in the event that the seal on the magnets was lost.

Basically, I see reversing the whole set up as having advantages.

Placing the gasket on the deck lip puts it in a high wear/tear position. The position of the gasket, inside the magnet spacer ring, is key.

By using magnets pre-sealed in epoxy from the manufacturer and sold as saltwater proof, along with
a second sealing under glass, totally protects the magnets from damage.

Perhaps you don't understand, the placement of the bead and the gasket relative to the magnets create a unique seal.

Test your ideas.

rob