Carbon fibre...
It would take lay-ups the size of those 'Gossamer' and 'solar-flyer' aircraft, but I suppose you could build frames from carbon-fibre and resin composites. Absorbent panels over the motor pods, shielded mufflers, larger, slower ducted props to reduce the noise footprint...
IIRC, Zeps had air-filled ballonettes to allow for expansion of their lifting gas. The vents were for descent...
Snag is that, once spotted once, the element of surprise is lost. After that, like those NarcoSubs off Mexico, it is cat & mouse...