I'm surprised you haven't had more responses to this, but luckily I've got a fair bit of experience on the subject having built loads of compact machines for customers.
The first question I ask is "Why do you want a deeper control panel? All the components and interface are safely contained within the X-Arcade unit so it does not need to be deeper. Add the fact you will need some kind of 'shelf' to mount it to and it'll be thicker/deeper anyway. As for space, check out our standard 'Mini' MAME cabinets for an idea of real space you need. The third example is typical, at roughly the same height as your cabinet plan, 20" wide x 25" deep. The control panel surface area is 18.5" x 6", with a cp section depth and 'height' of 5" x 5"....


To save space, you could do something like the first design (slim with sloped front and rear for free-standing) or the second (slim with sloped front but flat back for standing against a wall). I've attached a similar diagram showing how you could make the cab adapted for an X-Arcade, and a pic of the prototype I did for my nephew a few years ago...

