"I struggle to think of a material strong enough to hold an almost-vacuum yet light enough to lift off!" Ah, no the whole point of the PERB is that it doesn't

"I struggle to think of a material strong enough to hold an almost-vacuum yet light enough to lift off!" Ah, no the whole point of the PERB is that it doesn't need a near vacuum -- just as much partial vacuum as is required to reduce its net mass to zero at the altitude where it currently is.