Before I say anything, know that I've never mounted a monitor on a diagonal.
That being said, I don't think you have to take it out of the casing unless you want to. you could build bracing inside the cab with 2 (or more) beams going across the machine. These beams could be at the angle you wanted, and you can rest the monitor on there. With some careful measurements and deciding where the monitor should rest on the beam, it should work just fine. You'd need to support it well of course, but I'd think it was more than possible.