I am looking for the brand chairs of my buggy. The round rings (see photo) are broken, and I was wondering if they are still for sale ....

How big are the rings?

Could you adapt something like Subwoofer ports to fit?

thank you for your reaction Dave  .
I do not think this is going to fit. That ring is on both sides. on the front and on the back of the seat....

No idea on the make of seats but to replace I would measure the ring and try to adapt something it's amazing the sizes waste pipe fittings come in

Maybe these pictures are more clear:

Search for 100mm heater duct - various trim pieces and sections for going through different thicknesses of walls

You can get them in plastic and metal - a trim piece either end and a very short bit going through the seat

THANX   Dave , i going to search the internet/webshops.....