I've put together a box with sides as cut-away as possible so that you can see the mechanism at work. Triangular paper tubes help keep the structure surprisingly strong.

Rather than a circular axle  running through cut out circular holes (left) I've opted for a rolled tube fitted to the top of the body with a square pin running through it. (Right) This has the double advantage of reducing the number of circular holes that need cutting out and square pin fitting nicely to the arm top.

End view of the crank.