https://github.com/meehow/privtracker
In the past I‘ve just used some open tracker URL for the torrent announce (https://github.com/ngosang/trackerslist) but while these don’t allow searching for other users, they are still not 100% private even if you enabled the „private torrent“ option in your torrent creator.
With PrivTracker you get a unique private tracker url to use. I know you could also just use qBittorrent‘s embedded tracker (which is disabled by default!) or your friend could just enter your IP address and port manually (adding a peer with IP address) but this makes sharing fully private torrents much easier & convenient.