This template helped a ton -- thank you so much!
Also, for anyone who needs it: This might just be me, but there can sometimes be a slight issue with it getting stuck on the loading screen or the keyboard being unresponsive on itch.io after going full screen with itch.io's full screen button. It does fix itself if you click on the unity game (focusing the iframe), but if you don't want the player to have to click on it, simply add:
window.focus();
in the index file after line 98 (At the end of the OnResize() function). This fixed the issue for me.