How about we cut a window shutter, then grab some bamboo skewers, and sewing pins. Razor blade, file, little bit of glue...
Oh yeah, let's do an aluminum full-moon hupcap. Get some sucker to drink a big can of Monster with the screw cap, be a weirdo that asks for the cap or get out of trash, discard everything that's not a moon disk.
Push the window in at the top, it pivots open and it holds open for ventilation without a prop rod. Push it closed and the inner sill stops it flush.