First: WHY are you replacing the carrier? If it's not loose, broken or damaged, just replace the GROMMET (which acts as the seat).
If the CARRIER is replaced, it must be coated (outside) with permanent loctite, driven into place, and then strake the adapter plate in 4 places to prevent it from coming out.
Only ones I've had to replace were due to a new adapter plate, or someone's failed attempt at removing the old one.