Using what we had on hand, DH and I removed the window, covered the frame with visqueen (thick plastic sheeting), and then put the window back into place (it sits in a sill DH made for it when we originally installed it and is held in place by a piece of wood on each the top and bottom, that swivel.)
That quick fix lasted, until this week. The visqueen had apparently been through too many sunny days, windy days, rainy days, snowy days, hot days, cold days, and it gave out. On an extremely windy day, I went out to find it in shreds, and my chickens once again vulnerable to the elements and four-footed chicken killers.
Not having DH home to help me with another quick fix using more visqueen, I called on my Marine Mom skills to adapt, improvise, and overcome. I dug into my bin of old sheets that I keep around for tossing over the plants in the garden when an early frost threatens. I pulled out a fitted sheet that was just a little bigger than the window frame. I removed the frame, put the sheet around it, then stuck the frame back into place.
Voila! Rain can't get into the coop (well, unless it's really driving hard), and neither can coons or possums. And the chickens, while they don't get a clear view of the outdoors, do get a custom drapery for their window until DH gets home and can help me with a more permanent solution.
my temporary solution
Love my quick fix? LOL