You should have a couple of small screws and a couple of right angle pieces which mount threaded feet. These right angle pieces screw to the side of the 421 and then the threaded feet are screwed up and locked with lock nuts to butt up against the back of the mounting panel. Doesn't sound very secure, but in practice I have had no problems in the past. The only issue is the price of the things
It does sound as if you are going to have to make up a blanking panel in order to securely mount the 421.