I'm building my control panel first too.
I figured it the other way round. My idea is that it's the most complicated part of the cab (I'm doing one with replacable panels) I thought I'd better first get that right before I build the whole thing. So I'm first building/prototyping an integrated "monitor shelf/control panel/control panel docking bay/monitor cover glass holder". The cab will then be build around that.
In fact I, started out building a prototype control panel and I was unable to get it built the way I wanted it (I had figured on bending the lexan in an L-shape and was unable to do so) I'm glad I tried that idea out first. Now I try my ideas for swappable panels and if that works it goes in the cab, if not I think of something else again.
Besides having a control panel first gives you the chance of testing the diemensions of your cab. I found that it's pretty important to have the monitor and CP at the right height (which depends on personal wishes I guess) The best way to figure that out is to test on a mock-up for a couple of hours
