The Blockstream Jade is pretty cool with its camera and tx signature thing, but high res cameras at certain angles one wasn't aware of could potentially capture the transaction and give away something about it.

A better system would be a close contact infrared system that works like a small socket. No signal would leak from it at all, in any direction, not even a radio signal, especially if the shielding was carefully placed around everything but the IR emitter/sensor.

It would be possible to make the device completely resistant to EMP damage too, with the help of an IR translucent thin metal shield on the IR tranceiver.

Though we are in big trouble if EMP is a serious risk in general. But this device could be pretty well safe from such damage, and associated key loss.