Check out @ddmone’s post above. That’s called a NATO strap, and they’re available in a huge variety of colours, including basic black. They have a clever design that allows them to be installed and removed without taking out the pins. Pick a width that matches the pin size, cut to the length you want, and heat seal the end.
The double rings shown under the watch are standard, but only one of them is essentiol. It’s easy to cut the other off if you like a cleaner look.
