4 ways DC power connector KPJX-PM-4S mounting method
You can use standard 1mm stainless (what you use depends on the stress it will need to withstand).

The actual cutouts are fairly simple:



There will be a circular cutout for the main body (I have not downloaded the 3D stuff - available on the website) - the specific information is not on this print. Make sure you have at least 0.5mm oversizing. This print is not clear as to whether the keyways are exposed at the outer main connector body.

The seating plane is where the vertical red lines are.

Drill two holes based on the two screwhole centres from the drawing, suitable for self tapping screws (the recommended method, see above) or use a nut and bolt combination - the hole size depends on the specific screw you use.

There is a note on the length of the screw to use.