Bo,
There was an MB stencil for this some years ago; of course it is NLA and most MB dealers and body shops no longer have it.
Periodically, on the internet, someone is selling their own version, improved, made of silicone rubber.
The challenge with either stencil method is two-fold. First, in order to efficiently "shoot" the wheel covers with paint, you need to have all done at the same time...this means you are either horrendously inefficient--one at a time--or you need 4 stencils. Hmmmm.
The second issue, perhaps more important, is the fact that you get a cleaner masking job when done by hand. There is liquid mask as well as tape mask material, the painter's choice. The three separate shops I've had paint wheel covers wouldn't go near a stencil and they all knew about them. And yes, I've got a very good paint job on them.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored