I know seems silly.

found a 2din case that can fit a mini-itx inside.
Actually, now there are pc that are smaller than mini-tix (like NUC or others).
A possibility is to fit all in a 2din (maybe will be too hot), other option is to just place a screen on the dashboard and place the main system under dashboard or under front seat (just need to wire the video and usb cable).

^_^