hey try using two pieces of half inch ply but do not caulk them it wont do much good in the end take liquid nails apply to one board clamp togather for the posted time limit make sure you cut the boards out the shape of the window sill and then make sure it fits tight and take a roll of ducttape and tape around the board on the seam between the two it will minimise damage and cost less then a hole setup and frame and watever...ive don it plenty of times in my house...