From what I can tell, there are 3 options:
1. Mount a permanent wheel - this isn't an option for you obviously as your CP is already built and crowded.
2. A clamp on wheel - most of these require about 1-2 inch overhang for clamping. I'm sure they vary on how much CP real estate they take up. I also don't know what would happen if they rested on top of buttons or trackball. I would imagine that would mess things up. I tested it on a disconnected cab control panel (when I was figuring out how much space I would need), and it rested on and was pressing down several buttons. Again, my wheel went 6 inches in. You could maybe, maybe, cheat it up .5-1 inches, but that still means you need about 5 inches. I wanted it flush though. You'll have to look around to see if maybe there's a plug in wheel with a smaller footprint.
3. The TT2 mini wheel - you already ruled these out. I researched them and really like how they looked, but I wanted a pedal/shifter rather than use the buttons and could get the Drive Force Pro for about $45 from Amazon, which was worth it.
I think those are your only real options, unless you built a swappable panel or a dedicated wheel panel that could fit over your existing panel (which is something I thought about doing before finding these other easier options). Good luck!