on-boot-script is a popular method for extending the UniFi Dream Machine to execute init.d style scripts on boot. It works by installing a custom systemd service into the unifi-os container, but this method doesn't work on the UXG-Pro. The bootstrap.sh script below allows similar functionality to work on the UXG-Pro using a different approach (modify the Docker entrypoint script in the main uxg-setup container).
To install, either copy bootstrap.sh onto the UXG-Pro and execute it with sh bootstrap.sh or execute the script directly with curl -LsS https://gist.github.com/joshuaspence/a745426ab7d72631b1a4313f15e1774a/raw/bootstrap.sh | sh.