37mm Watch Case Wrench (30.75mm jaw width) (Back Opening Tool for SNK809)

thingiverse

This quick fix on the Chuckdotdot case tool does the trick. I just tweaked the wrench faces by 1mm each, boosting the inner diameter (jaw-to-jaw) from 28.75mm to 30.75mm across to fit my Seiko SNK809 watch's 37mm case perfectly. It works flawlessly! If you're opening your case for the first time, be aware that it screws in very tightly, so handle with care to avoid crushing the o-ring or letting dust get inside when working on it.