I think he means a square cover like a humbucker
You could make something yourself?
Is that a model one in that Precision...? saw them often enough but just don't remember that one, might have to dig through my pics to see as I have a quite a few hundred pics of that era WS...
So....why not use a Fender cover on a Fender-style bass?
Or do you mean an ashtray with the 'Fender cover'?
Shouldn't be too hard to get measurements on whichever cover you like.
Are those the Jazz Special pickups with the larger pole pieces? I was looking for some of those.