How do I hang curtains on a sloped ceiling above a recessed window (no studs)?

Hi all,

I’m trying to install a curtain rail above a recessed window, but the ceiling above it is sloped. The surface seems to be either plasterboard or some kind of thin soft wood panel — and I can’t find any studs/joists where I need to mount it.

Because the ceiling is sloped, I’m worried about the downward/sideways force on the rail once curtains are hanging and being opened/closed.

Has anyone successfully mounted curtain rails on a sloped ceiling like this?

Would heavy-duty cavity anchors be enough? Or should I install a timber batten first to spread the load? I’m planning to hang blackout curtains

Previous owners had curtain rail screwed onto the window frame

Any advice or examples would be massively appreciated!

Thanks 🙏