In similar situations I have seen long springs like the old screen door springs used. All you would need is two small hooks,( 3M?). One on each side of the window to hook the spring to. You could do the same to hold the bottom down. Our 2015 13 Scamp has a curtain rod at both the top and bottom for the door curtain. Bungee cords could be used in place of springs too. You can buy shock cord by the foot at a hardware store. Use the little metal crimp clamps, also available at the hardware store to form a small loop in each end to put over the hooks on the door. That would look nicer than the big bulky regular bungee cord hooks. Other folks have put decorative contact paper on the door window in place of the curtain which would also give you privacy. But we like to be able to look out that window sometimes. Just some other ideas. Good Luck