Yep, both Bradderz and Scott speak the truth!
I was looking into doing it with my RS4 Rep bumper until an oem unit became available and i went that route! The concept is, for all washers other than oem RS4, the washers unit/caps are pushed up with the pressure and then rest back on the bumper itself, so in reality you only need a hole big enough for the washer jet to get thru, but less than the width and breadth of the cap so the bumper still supports it when retracted!
I still have my original Sport bumper here with the holes, i can measure sizes if you need it but im sure it'll be obvious when you start doing it!