You'll find constructing fly screen for this type of window slightly more difficult than a standard window. You will need to work around the winding mechanism as the bottom ledge is not flat. If you were to fit a standard screen, there would be a gap on the side of the winding mechanism. At my property, someone has pop-rivetted a Metal Mate 20 x 12 x 1.4mm 1m Aluminium Unequal Angle to the bottom of the screen and cut out around the winder to fill the gap. I've included an illustration below of how that would work.