The Blue Sea USB outlet I installed took a few minutes with a round file to enlarge the existing 12V outlet hole. The outlet's exterior trim is oversized, so while my work was neat it didn't have to be perfectly cut. The Blue Sea instructions note the hole diameter required -- 30mm, IIRC-- but since I only mounted one outlet in an existing hole, I didn't pay too much attention. The Blue Sea outlet comes with a rectangular trim piece with two screw mounts you'll see in some photos that added nuthin' to my installation so I set it aside. I'm not sure what is the point of that additional trim piece.
Don