Here's a post about how I solved that problem for my 2005 Bambi. (Never mind the air conditioner). Just look at the second picture and you'll see the shroud I made out of a quarter inch sheet of plastic to cover the frame for the AC. I taped a sheet of wrapping paper over the window opening and traced the outline for cutting the hole. (If you can't find a sheet of plastic, Plexigalss or Lexan should work also.) This fits very nicely inside the existing
groove in the window frame. What is not shown in the pictures is that I put a strip of Velcro around the inside edge of the shroud, to which I stick one half
of those expandable screens that people put in their house windows for ventilation.
It doesn't need propping up because the window won't close all the way with the shroud in it. If I can find a picture, I'll post it later if you're interested. It really works out quite well getting cross ventilation across the bed, especially with the fan on.