So, the crate mechanic is cool, but it makes it way too easy, you can literally just push the crate in front of the entrance and keep it there the whole night, thus eliminating one of the 2 monsters
So basically all you need to do is press the button when you hear the music and refill the oxygen, and somehow I doubt it should be that easy
You could make it so that theres a 3rd monster that breaks the crate, or, you could make the oxygen go down really fast if the crate is covering the entrance for so long, idk