For 9 days a year I'd leave the rear window open a bit and tape some plastic over the gap to keep the dust out and wear earplugs.  Tons cheaper than electric valves.  Either that or replace the lower rear window with plexiglass and cut grommet holes in it.  Or steel and mount quick release fittings.  Replace the new window with the old glass one when done. This would probably be easier if you have a bolt-in lower window.