Curtain rails for plasterboard dormer

[Ianthomas79]

We have a newly constructed dormer window and I’d like to put up a curtain rail / pole - but due to the nature of the construction, am struggling to see a way to do it.

The dormer is a timber framed construction, with all the structural members buried under 50 or 75 mm solid foam insulation. The plasterboard is directly in front of the insulation, so there is no gap for plasterboard-cavity fixings.

I’ve attached a photo of the construction showing the insulation, pre-plasterboard.

Any suggestions as to how I can attach a curtain rail or pole to something of sufficient substance are most welcome!