9 speed MTB on 11 speed direct drive turbo

[ukracer]

Hi ,
Looking to put my boys 9 speed mountain bike on my direct mount smart trainer to allow him to use Zwift etc .
Thinking of my options to make the swaps as simple as possible
What would you guys do ?
Cassette off mountain bike wheel swap everytime or is there a smarter way to go which allows me to keep the 11 speed on
Thanks

[Singular]

The "best" option is to find a cog/gear that lines up reasonably between the two (ought to be the middle) and use manual resistance/erg.