I was thinking of using one of those plastic / polythene boxes pre-recorded VHS Tapes used to come with (In my case – excuse the pun – size doesn't matter). I'm sure loads of kids / their parents will have one or two which could be 'liberated'.
Plenty of room for a Battery Pack, USB Hub, WiFi Dongle etc, cheap (I must have a couple of hundered in the attic), simple to chop about, non-metalic, reasonably rugged and easy to get into if the need arises. Slide a small solar panel into the front or back pocket to re-charge the battery pack, and Bob's your Uncle.
I'm going to call it a 'Punnet' (Portable Unit Needing No Engineering Talent)
EDIT: I was also thinking about super gluing a couple of Cable Ties to the base of the case and use those to secure the PCB – I'm not sure that will work, so will have to wait until I can get my hands on a Raspberry !
Blue Peter eat your heart out (LOL)