Fix curtain pole to plaster coving

[pepe]

We are looking to fit shutters and curtains to our bay windows. Shutters are fine they attach to the victorian window frames of the bay window.

Problem is we want to keep curtains too - rails are currently attached to window frame. Above frame is plaster coving which is not a flat surface, but is the only place available to fix to.

Any ideas on how to best achieve our goal?[/img]