Today’s strap is a ZULUDIVER (MKII) 400 rubber strap in black, by Watch Gecko, shown here on a Seiko SKX009 watch. The rubber is embossed with a carbon fiber surface treatment and it comes with a folding buckle that allows micro adjustments for a perfect fit.
Additionally, the strap is reversible for a different texture and you can cut it to length (the buckle hides the cut). Made by Italian manufacturer Bonetto Cinturini.
You can find the strap on the Watch Gecko store, and some of their other straps are also sold on Amazon on their Amazon storefront. As they sell many other straps, Watch Gecko is a good place for E.U., U.K. or U.S. strap buyers.