This may give you ideas. Using some treated scrap I built 2x4 frames slightly smaller than the window opening, then distressed some old 1x4 and etc. then hit them with a little stain.
I then screwed into the bottom corners two 3/8 hex screws making a sort of footing. By backing off the screws with a wrench they simply wedge against the window sill and header board. It takes me about 2 minutes to put them up or take them down and they stay put in any weather condition.