Making HTML5 videos stored on AWS S3 **difficult** to download (because I cant make it impossible)

I think using it with Lightbox makes chance of download hard. You can read this article: How to create a Video Membership Site with Complete Protection to Your Videos and after the content I found this: Limitting Accessing to Video Content

These are for Wordpress but I think you can easily apply for PHP or other web programming language.

Note : I recommend you research how YouTube can store their exact video URL and make hard to find!