I made a mock-up box at 63cm x 36.5cm x 15cm (6" thick/deep). This would be worst case scenario thick for the mount + monitor. After having received the mount, I can confirm it's 3" thick folded up + the monitor which will be 1.5-2" thick, so it should total only 4.5"-5".
I removed the bottom panel off of the arch. Here are some pictures of what's under there:
Now I need some ideas for how to bolt up the mount to the roof ribs there. The mount will have to be mounted slightly lower (2cm) than the ribs to match the mounting surface of the blue panel I removed. I was thinking something along the lines of:
4 metal screws secure wooden block into metal van rib
4 - 6 wood screws secure mount into blue panel + wooden block
Unless I build the wooden block around the rib to be thicker in front and behind it, I'll only have 2cm of wood to screw the mount into.