I bought a metal project box because I knew I would need to drill/cut holes into it for what I need. What I did was create a paper template from the circuit board I wanted to mount into the box. I did this by taping it to the circuit board and then marking the mounting holes on the paper in pen. I took the template off of the circuit board and transferred it to the back of the case. I used a punch to mark where the holes would need to be drilled.
ProTip: Make sure to tape the corners down before you hit the project box with your punch so everything stays properly aligned.
Here you can see the box with the template:
After I drilled the holes, I used the screws to check and see if the heads were going to stick out and they sure did. I bough philips machine screws with a flat mounting head type so I wanted these to be flush with the case as to avoid accidentally scratching things. In order to make this work, I used a counter sink. You can see what the holes look like after I counter sunk them:
After the holes were counter sunk, I screwed the nuts down to permanently mount the screws and test fit the PCB. Next I checked for space/fit for the RJ45 adapter with the PCB mocked up in place. In order to check the fit, I first cut the backplane bracket down to size: