I've just removed the old, terribly applied caulk around a window in my new house. The window has a bay sill, such that the wall on either side of it is at an obtuse angle to the window, rather than at 90 degrees. Some plaster crumbled and fell through into the hollow wall at the joint with the UPVC frame. Filler and caulk will also fall through. How can I repair this hole for caulking?