There is a task – to make the joint horizontal particle board, which runs along the entire wall from corner to corner, with this wall and the two adjacent (with which the board is joined by short sides).
The walls are crooked, to fit the chipboard will not be possible, there will still be gaps, how and what can close them?
With plastic or aluminum corners.
It's possible to make it fit. Why are they so crooked? The worst part is that any crooked line is going to be annoying. Can't you just plaster it now?
Use a narrow ceiling plinth. It is flexible, sometimes made of soft foam, which crumples a lot if you touch anything. And there is a denser material, but still flexible. Paint in the desired color and glue with liquid nails.