If you're hanging a net or voile within the window recess, your best option is to use a curtain wire or curtain rod. Thread the wire or rod through the slot top and position it in the window.
Wires have hooks that need fitting in the window. Rods by contrast either hang on hooks or use tension to hold in place.
If you want to hand a voile above a window recess, then we would recommend a narrow 19mm curtain pole.