You need to create a “fullscreen” output format and set its params in the config files.
This custom output format will have to live alongside harry-potter/index.html as harry-potter/index-fullscreen.html (or something). You may be able to have it live inside a directory as harry-potter/fullscreen/index.html but I’m not sure…
Then, you’ll have to set up your hosts as to make url rewriting from harry-potter-fullscreen/ to harry-potter/index-fullscreen.html
Study the doc on output formats and if you need help on those, hit the discourse with the info you missed from the doc!