I'm a big guy with a big wrist. I got a watch to watch my blood pressure but the band was just slightly too short (120mm) so I made this 150mm which is more than enough for my 10"/240mm wrist.
The band the watch came with uses quick release pins which a great and not that hard to pop out. I put a slot in this for the lever and it works perfectly. If you don't have quick release pins, this would still work just fine.
I was just going to order a new band online and I made this just for fun to see if it would work... and it works so well, I think I'm just going to get some black TPU instead!
Obviously you'll need to use a very soft plastic like TPU or Ninjaflex. I used Yoyi TPU as it's all I have and with my limited experience of 1... prints well.
Note, if people want, I can easly make the band wider or narrower, I just don't know the quick release pin hole placement. It's probably similar, but I don't know for sure. Length would be no problem, but this will fit some pretty big wrists, so no idea if that's necessary.