@Essie274 I have had success with draping towels /sheeting over an open window edge .Secured by cliping material on to a skirt/trouser hanger with the hook over the window .
It really does make a difference to cover the outside of the window if you can .Although my method means you'll need to remove the hanger so that you can shut the window when it's in full sun .
Also if you have a room not in use close window and the door during the day/when hottest .