Don Stauffer:
One emergency fix if you push a window in and it is not too large- use Micro glaze to make a "replacement window." You will still have a piece of plastic rattling around inside the model, but the microglaze at least covers the hole. I have used Micro Glaze to make windows as large as quarter inch square
This is fine for windows on a flat panel, but if the window is curved, it's not quite as effective, as the surface tension in the fluid (which helps form the "window" in the first place) tends to pull it flat before it dries Hanging it in a position where gravity will pull the liquid down (outwards) may help, but it does shrink somewhat as it dries and will pull back..