This strap is 1mm thick and 18mm wide. Print it in TPU or other at least slightly flexible filament. There are three design variations; one with long buckle slots, one with diagonal slots, and one with small holes. 

To assemble, you'll need watch strap spring pins and the buckle off a Casio Calculator or similar watch.

First use the spring pins to attach the short and long straps to the watch. Then slide the strap keeper over the short strap. Finally, attach the buckle.