Wherever RJ45 / W-Lan and current do not go, the Raspberry Pi can be operated with a UMTS stick and a Powerbank.
There are some examples of application: software presentation, measuring data, Internet radio in the garden etc.
With a little manual skill and a Dremel Multifunction Tool you can mill-cut in the transparent Plexiglas front of the OpenDisplayCase Base. With Velcro Cable Ties (width 10mm) then UMTS Stick and Powerbank are attached.
The device is activated via the Powerbank button, the shutdown is implemented via a button on the Watterott RPi display B + 2.8 "and a script.