Here are two pictures. The first shows the digital display. It is small enough to mount on the 5-1/4 plactic knock-outs and still not hit the metal part of the case. Notice the retainer tab. That holds it in a cut-out.
The second picture is your standard 5-1/4 knock-out. Cut an opening in it just large enough to pass the display module and the retainer tabs will hold it in.]