Had some jacks behind moving cabinets that kept getting damaged from cables sticking out the front.

This is designed around the larger Leviton faceplates. Ones that say "3/8" larger", but by coincidence standard size also fits really well sitting inside the lip.

Use #6-32 X 2in screws to replace the shorter faceplate screws.

Prusa

i3 MK3

No

Doesn't Matter

As long as you can bridge decently no supports are needed.
There is built in tolerance for a little sag.