Periodically I wash mine in the dishwasher. Take the top rack out and lay it open (open side down) on the bottom rack. Make sure the heating element is turned off for the dry cycle. Take out and lay open on top of towels to dry. Usually takes a day.

The pressure of the water really cleans it well, and the water is hotter than I would use if I washed it by hand. Just use the regular dishwashing detergent.

My wife does this to baseball caps and shoes as well. They don't get mangled from the washing machine.