I liked the idea but not finding washers of the right size I designed the coin, with sides of beer and geocaching.
I have used washer of OD: 23mm and ID:11 mm
I printed both halves at the same time using Smartfil gold PLA, 0.2mm layers with supports. Enable ironing on all top layers to smooth the finish.
Use super glue to secure the coin's inset face, hiding the washer inside.
The author marked this model as their own original creation.