I included 6 keystones on the model as it gave them plenty of room with venting in the middle. I'm planning to install 2 ZimaBoards into this 2U case, so wanted to pass through the USB/networking jack, and a RJ12 for JetKVM DC control board each. 

I need to clean my printbed but presumably the face down orientation would be the cleanest. I had some trouble getting all keystones to click properly in the slots (PETG) but a zip tie to shim the clips open holds them tight. I might try PLA next time, or mess around with a different orientation/support configuration

The detents on the sides hold it well enough for me, it required about as much pressure as the metal ones that come with motherboards, but it is only a friction fit, and can be pushed inward with enough force.