Our new place has aluminum double-hung windows. We don't want the top portions to be openable, at least beyond a few inches, for both security and child safety reasons. (The bottom portions are barred.) How can we fix it so they can't be slid down? I don't know if it's OK to screw into the aluminum in any areas, either the frame or the sash. Also, if we put something into the outer frame to stop the window from sliding down, it would be easily removeable from outside. So at a loss.