A slight evolution of the design....
The pi sits on standoffs with small lugs on the to locate the pi board. longer standoffs from the lid trap the pi onto them and the lids is screwed to the base from the below using countersunk m2.5 machine screws.
A couple more small tweaks to make to improve the installation of the pi and allow a usb gps on extension cable out through the case and I'll post the STL files.
There is a disadvantage in having to open the case to access the ethernet or usb ports for software updates (though the sd card can be accessed externally) but for me this outweighs the advantage of having a single robust unit that I can take in and out of different aircraft.
I don't use the GPS dongle but case is large enough for one. Instead opted to use collisionaware/nmeagps route and so this case stays in my bag. usually behind the p2 seat with the 1090 antenna sticking up vertically and the tuned low profile antenna gets stuck to the rear windows with slot head suction cups acquired here
http://www.suctioncupsdirect.co.uk/ . The only cable to sneak between the seats in the audio lead to plug into my headset.