Since you did not state how much larger the old mount opening is hard to determine the space you need to alter. You mention by putting screws in at angle can be done that therefore indicates to me you have very little difference in space, perhaps 1/8"? what comes to my mind is why not take a piece of pine or similiar wood and glue it to the existing egde of existing mount on one end. Again I don't know the thickness of existing mount perhaps 1/4" or less but you could also add a piece of thin ply beneath the piece you glue in to span some under the existing mount. clear in my mind, hope you understand. If you were to make a new mounting board it seems to me the thickness of that piece would move the servo arm out too far in slot?