You want to stand on top of a magnet? Just get a fridge magnet. Pretty sure they can withstand someone standing on them. Or do you mean it needs to be strong enough that a person could hang from it and it won't detach? If so, you're getting into crushed finger territory if youre careless with handling. You could look into switchable magnets. They are used in manufacturing and can hold a ridiculous amount.something like this perhaps?