If it's the 'lozenge shaped base plate' I too struggled to find an exact fit (I'm assuming you have a twin wiper, single washer jet in the middle). I originally fitted a generic twin one on a home made plate, but was never happy with the angle of the jets, so I got a Jaguar one which covers the plate holes and works much better.
There is a spare wheel mount on ebay. Looks like a 'wire wheel' one that's been adapted to a disc wheel one. Should be easy to cut of the studs.
Wheel Mount
