My cab is/was....
After a couple of false starts, I found it easier to start designing the cab from "known" dimensions. For example, rather than try to design that ultra cool cab only to find that the monitor of choice isn't going to fit, build a "box" to the dimensions of the monitor and design the cab around it. For a lot of cabs, this isn't really necessary as there's usually plenty of space internally. But for some, such as bartops or narrow uprights, this is something to consider. This is especially true for cabs where Control Panel space is a premium even if there is plenty of space internally for the brains.
Pay special attention to which side is "up" and which side is "down". The default white and gray (or blue and white... whatever). There's a difference and if you don't get it right, it'll bite you in the ass at the worst time.
Remember that there are two different measurements for lumber. One is "exact" and the other is nominal. Your classic zombie smashing 2x4 board is not actually 2" by 4", but rather 1-1/2" by 3-1/2" or something along those lines. 2x4 hasn't been a true 2x4 dimension for years. On the other hand, your 3/4" MDF is pretty damn close to 3/4". This makes a difference for some. Doesn't matter one lick for others (Ond cuts his own trees and fabricates his own lumber

)
Take advantage of Sketchups group or model feature to divide your cab into realistic cuts. There's been a few times I designed what I thought was a really cool cab but realized I couldn't cut it correctly using the tools available to me.