You can make screens that fit IN the windows
with the "make-your-own-screen" materials at the
hardware store.  That is, you buy channel material,
corners, cut the channel, make the shape, then
put screen into the grooves just like other
household screens.  You can cut/file an upper
plastic corner for the curve in the window.  The
channel material I bought fits perfectly in the
sliding window channels.

You open the window, put the screen in the goove
and then close the window to hold the screen in.
I made a pair for my '81 non-Westy when we used
to camp in it.  I made them almost as big as the
max window opening for max ventilation.

[The Westy windows are designed to be secure in
the first notch and the Westy screens work with
this.  Screens as I describe don't do that for
either Westies or non-Westies.]

                        Richard A Jones
                        Boulder, Colorado