Ceiling diffuser fixing

[nwxh]

I intend to use the linked ball jet diffuser in the ceiling to replace a very old (maybe as old as 1972 house) warm air vent. I'm not sure how it's meant to be affixed e.g. does it just rely on being wedged in to the plasterboard ceiling? Interestingly, the outer ring cover unscrews and comes off but the part behind which butts the ceiling (from beneath) has no screw holes and a screw head might foul the ring being reattached anyway. What is the best way to affix it securely?

 

[foxhole]

I intend to use the linked ball jet diffuser in the ceiling to replace a very old (maybe as old as 1972 house) warm air vent. I'm not sure how it's meant to be affixed e.g. does it just rely on being wedged in to the plasterboard ceiling? Interestingly, the outer ring cover unscrews and comes off but the part behind which butts the ceiling (from beneath) has no screw holes and a screw head might foul the ring being reattached anyway. What is the best way to affix it securely?

Fixall

 

[jj4091]

Looks to me like the moulded piece at " E " is a clip that will secure it to the plasterboard by just clipping it in.