I need to share a project and I want to know if there is an easy way to secure the web player so you can’t find where to download the .unity3d file from if you look in the page source.
I searched a lot about this subject but I can only find security information about protecting from decompiling and extracting assets.
Nope, only thing you can do it obfuscate code.
Something like this: CodeGuard | Utilities Tools | Unity Asset Store
They will still be able to get the art though, like models and textures. 
Theoretically everything possible of course, but that would include smart data streaming from server, but its still by-passable. Even triple A companies cant protect themselves.