I haven't written a guide on how to do it, but basically the idea is to set up a reverse proxy for apache by nginx:
Switch apache to use different ports. I.e. 8080 and 4433 for http and https by altering both ports.conf and sites-enabled/default-ssl.conf
Then create a new vhost for your main domain on nginx and reverse proxy the entirety of traffic from apache 8080 -> nginx 80 and apache 4433 -> nginx 443.
Now, the server name config will differentiate the traffic. Traffic destined for yourdomain.com would end up at your quickbox panel and traffic destined for plex.yourdomain.com would end up at the plex proxy.