cut a piece of strong card just bigger than the hole, fold it in half and put a hole in the middle, put a piece of cord through the hole and tie a pencil on the end of the cord, then put the whole lot through the hole and pull the cord so the card is flat against the back of the wall with (hopefully) the pencil at 90 degrees to the fold, then just plaster over the card (you may need to tie the cord to something to keep the card in place). when your done just cut the cord and fill in the small hole left by the cord,