In our "15 Windsport 27K we have a similar window which came without a screen and no way to hold it open for ventilation. Our window consists of a large lower window which is hinged and a fixed narrow upper window. What we did was to buy cloth screen material , sew a binding all the way about 1/2" wide and then sewed 1/2" Velcro to that (the soft side of the Velcro). On the window frame we attached the the other portion of the Velcro with double sided tape. To hold the window open I cut a 3/8" rod in 6", 8", and 10" lengths to hold open the window. To open the window we just pull open the lower right corner enough to reach the lever, open the window and place the rod between the frame and the window. The window is heavy enough that the rods stay in place. We've been using this arrangement for the last 5 years. Good luck.
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