Restrict FTP Access to a single WordPress Site

I have a Ubuntu 22.04 droplet with several WordPress sites. One of them requires outside technical support with FTP access.

I am able set up a user account for “techsupport” with password authentication, which can be used for FTP access to the entire droplet. But I want to restrict FTP access for this user to a single site, e.g. “/var/www/onlythissite.com”

I have tried, unsuccessfully to adapt these instructions to my situation:

https://www.digitalocean.com/community/tutorials/how-to-enable-sftp-without-shell-access-on-ubuntu-20-04

By manipulating ownerships I am able to keep the tech support user out of other sites but not the rest of the file structure:

sudo chown -R techsupport:www-data /var/www/onlythissite.com

But so far I am unable to do what I really want to do, which is to restrict access for this user to this single site, while still permitting it to function as a WordPress site, and preventing access to the rest of the file structure.

Any ideas?