Where the ring is not big enough and you have difficulty in locking the ring, you can use this extension bracket . It is very easy to fit. Please check that it is tight and you can also gain extra security by spot welding it or putting a spot of strong adhesive paste on the screws.
The blue film peels off after its fitted.
The locking part is best kept at the bottom to allow for water drainage and also keeps the extension bracket out of sight. Please check frequently and ensure it is safe.