Fixed region/dimension screen snipping tool
You can use the screencapture tool, part of macOS:

screencapture -x -Rx,y,w,h /path/to/capture.png


x,y is the coordinates of the top-left point of the rectangle,
w,h is the dimensions of the rectangle,
-x mutes the capture.

You could wrap this in an Automator workflow or Keyboard Maestro macro and customise it to your desired output.