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7 4-2 tall lego bricks
1 2-4 flat lego bricks
Cut out 6 of the 2-4 tall lego bricks.
Put the whole 2-4 tall brick upside down and glue the empty ones on top of it.
Let the glue dry for an hour, then stick something in the safe. Then stick the 2-4 flat brick on the hole.