I used a standard sheet of plexiglass cut to the size of the window and I made the hose mount out of 1/8 draft board ( would be less glue with 1/4 inch). The hose mount is inserted through the hole I cut with a Dremel tool. The plexiglass was attached to the window opening with white strapping tape because it matched the white window. (Would need a more clever solution such as Velcro if you want to be able to remove it)
Super easy and for me because it was inside the actual window when closed, I don’t have to worry about elements. But you could paint the mount and seal it to make it more weather proof.
The issue I would see is if your window cranks but the physical window is both inside and outside the Frame. That would require a different solution. I did see those in my research, but I didn’t need them.
All parts at any hardware store. Cutting with dremel did require having a dremel but I could have mad a section with the GlowForge, but then I would have had to piece that together with larger plexiglass. Since the large sheet wouldn’t fit