On our first floor we installed 2 inch door latches on the window frame for security. Works as a window stop and no one can lift our windows higher than the latch. Yet we just retract it if we want to open the window all the way.
How to Decorate with Dollar Store Items - 18 Ideas!
Cut a semi-circle from a large piece of recycled cardboard to the size of the window, cover it in fabric (stripe to the interior and white to the exterior) and fit it into the window. Then make a typical roman blind for the square portion of the window.