Cut a length of timber the width of the window frame (or slightly wider if necessary), run a router around the ends and top to make it look pretty, then sit it on top of the window frame and mount it to the studs. That’ll give you a nice solid point to attach the curtains to.

Edit: Going the whole hog with this idea and extending a box out from this mount to make a pelmet for the curtains is also an option, and one that will reduce energy loss too if you make it suitably resistant to air flow.