This is a remake of a model I saw somewhere (sorry, can't find it anymore) but it did not have an option to fix the clip to the window frame.

It has a angled part so you can mount it to the side of a window frame and prevent it from rotating. Altho, with little usage and properly screwed down, it should not move much. 

When the window frame does not suit the angled part, you can clip it and use the hole in the front.

I printed mine in PETG at 20% infill for optimal strength and flexibility. I tried one of the prototypes for durability and I could unfold it completely flat multiple times without breaking or distorting.