This is a version oft the famous Stöpsespiel.
The number of player is unlimited. At the start every player receives 6 plugs. The task is to get rid of your plugs as fast as possible. The box has 6 number holes. One the hole number 6 is connected with the inside of the box. A plug placed here will "disappear" from the game.
Use a dice to determine the staring player. The one with the highes value starts. In case of a tie repeat.
The player rolls the dice and places one of his plugs in the hole that matches the number. In case the hole is already occupied the player can't put his plug there. Instead he has to take the plug from this hole. The player can repeat rolling the dice until he either has to take a plug or doesn't want to roll the dice any more (You have to roll the dice at least once). The dice is passed to the next player.
Winner is the first player without plugs. You can continue playing until every one has no plugs left.
You will need 12 cubic 3mm magnets and a dice. Glue the magnets in the hole in the base and the lid.