When I finally got round to doing some more work on this box, I decided to make wooden floor. It is made of lolly sticks and takes some patience.
Then I painted the floor with diluted white paint.
I also added proper skirting and cornices. This was a part of my ongoing project of making proper skirting in all rooms and houses - a separate story.
After this, it felt better to start putting in things. Wallpaper and floors make a huge difference in a project.