Seeking advice on how to finish a built-in cubby that I found in my house I was lightly renovating my bathroom (changing out a medicine cabinet) and when I took the cabinet off the wall I discovered a semi-finished square hole in the wall that looked like the housing for a small, long-gone built-in medicine cabinet. It’s kinda cute and I have this idea of finishing the interior and keeping it there as a hidden shelf behind the mirror I plan to hang. I also would love to hang the mirror with self-supporting hinges so you can lift it up and get to the cubby (it is a 20ish lb mirror about 30”x40”) and I don’t know if that’s realistic or what kind of hardware options I have. The 4 interior sides are all wood, so I plan to clean them and use wood filler to smooth those walls before priming to paint. But the back of it looks to be made of some kind of durable brown paper. It has a seam that needs to be filled-in before I can paint it. But what can I use to finish a paper wall? I tried to google the answer but I was mostly getting info about installing or removing wallpaper. (which isn’t relevant to this project). I don’t wanna place any drywall if I can help it, I’d like to just use some kind of filler to smooth out the seams on the back wall and I think primer would seal the surface for porosity. I was thinking of using wood filler but will wood filler actually work on paper? It’s unclear to me what would work. Thanks in advance for your advice!