Gluetun should be able to do this without any extra tools, check out the qbittorrent example on the gluetun wiki: https://github.com/qdm12/gluetun-wiki/blob/main/setup/advanced/vpn-port-forwarding.md#qbittorrent-example

I do something similar for transmission, though I put it all in a script that I mount into the container so my command is just VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c "set-transmission-port {{PORTS}}"

I have the qbittorrent web UI running in a separate container that's attached to my gluetun container. I use the VPN_PORT_FORWARDING_UP_COMMAND option in my gluetun container:

    environment:
      - VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c 'wget -O- --retry-connrefused --post-data "json={\"listen_port\":{{PORTS}}}" http://127.0.0.1:8072/api/v2/app/setPreferences 2>&1'

You may have to adjust the http://127.0.0.1:8072/ part based on your setup.

Why not use a provider like AirVPN that lets you use the same port number all the time?

Like the other two comments are saying: use a recent version of gluetun and its built in functionality.