Nook/Built-in Bed, How to go about it?

Well, I'm moving into a barely finished attic that I can't even stand up in.It is extremely tiny.In an effort to make it as spacious of a space as possible, I'm taking advantage of what is already there. There is an elevated part that would be perfect for one of those built-in nook like beds. You know, the ones in random wall spaces. Does anyone know how to go about making one?
My main concern is what mattress to get, my queen is never going to fit. I have no idea even if a twin will. I was either thinking maybe a futon set-up or daybed mattress, or even a foam one. I have no idea. I also wanted to find some way to attach cushions to the wall due to the slanted ceiling.

If anyone can offer advice, please do!
Added (1).

^ this is basically the kind of thing I'm dealing with
>>> Nook/Built-in Bed, How to go about it?