I've tried to get a collector box/link pipe gasket from the usual places (D Silver Spares, Lings etc) but the item is discontinued by Honda and there doesn't seem to be any stock left. What do other NC24 owners use instead as a workaround?

Silicone

I used very thin stainless steel foil wrapped around the inner pipe until the required thickness is reached then slipped the outer pipe over it and clamped it tight. worked a treat. You can get some from auto parts shops with a sticky backing on it which would stop it unwrapping when you are fitting it. Car enthusiasts use it for heat reflecting/protection in and around the engine bay/exhaust areas.