Buy a replacement for the original Boogie Cruiser from the built-in-the-USA bike company, Boogie Bikes.  This Nut directly replaces the factory installed nut and comes with a dust seal to keep contaminates out.  

Specifications:

Weight: 1 Oz

Dimension Across the Flats: 1 & 3/8"

Origin:  China

Tube Seal: 25.6mm  /  1 & 1/16"