If the police wanted to fabricate the silencer, they just needed to do another official search and pretend to find it. They didn't do another search and were given the silencer by the relatives.
If the relatives gave the police a silencer with blood, hair and paint on, it is certain it was used for the massacre. So the police did not need to fabricate anything. They even lost the hair, which was almost certainly Neville's. Which shows how uncommitted they were to frame Bamber.
Surely people are not suggesting the relatives gave the police the silencer with nothing on, then suggested the police fabricate evidence. The police being happy to oblige.