Hi @Daisy2069,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you join us.
Have you stuck your hand up through the hole in the plasterboard to see if there is any framing behind the plasterboard?
If installed correctly there should be timber framing just back from the lip of the plasterboard. For the duct to stay in place, it will need to be screwed into timber, otherwise the weight of the duct will pull the screws out of the plasterboard over time.
If you find that there is nothing there, you'll have to install some timber, so the screws have something solid to go into. I'd suggest using 70x35 pine shorts as they are cheap but sturdy.
If possible, these should be attached to the ceiling framing, but if not, they can be screwed in place with 7G x 30mm Plasterboard Screws up through the plasterboard. This is a technique that is commonly used called back blocking.
Ensure you place the timber at least 25mm back from the lip of the opening to ensure the clips are not fowled and the plasterboard does not fracture.
The next question I have, is what length are the screws you are using?
Plasterboard in residential applications is usually either 10 or 13mm thick, and as the flange on the air duct looks to be reasonably thick, I'd suggest the screws you use are at least 40mm long. Something like these 8G x 40mm White Button Head Screws will do a great job and not stand out too much.
Ensure you get good connection to the timber above, otherwise they will also pull through the plasterboard over time.
Let me know if you have any further questions.
Jacob
