How about instead of a tablet device a literal metal tablet with the QR code engraved in front (and 12 words on the back with a covering).

That way it's guaranteed to be 100% offline.

The mobile app is probably good enough for a spending wallet for now... but I'm sure in 10 years things will change a lot. Wearable tech, VR/AR will progress. There will be something better and more intuitive.