First off, congrats on the MkII - its a great watch.
You’ll need a 20mm strap with straight ends to fit that watch. I’d recommend rubber and not silicone as the material. Silicone tends to be a lint magnet.
ISOfrane is the classic choice, but expensive. It comes in 20mm and a variety of colors.
ToxicNato has a similar strap in a similar material for much less money.
ChristopherWard has a nice version that looks less of a dive strap. You can find it here:
https://www.christopherward.com/strap/20mm
A better picture of it is here on one of their watches