Not hard and the glider needn't be special; most already have a stunning glide ratio. I've been up in lesser wave in a clunky old trainer. You do need to coordinate with ATC to keep separation from jets. And the oxygen rig does has to be more serious than a nasal cannula above 18000 ft.

Of course then there's these guys going to 90000 ft ... https://perlanproject.org/

As a glider it's probably easier with ATC to stay under 29,000 (RVSM), yeah?