Well, I ran mine without the joint for ages.
Got a spare cup off Paddy and got a fire ring for a Vauxhall in the end, which seemed okay in terms of fit. Tis fine.
You can email Supersprint and they will help:
'Hello.
The i.d diameter of that ring is 50mm.
If you are in a hurry, you can find a replacement part at any Opel/Vauxhall
dealer in you area, or at a local muffler shop, since it has the same i.d.
dimensions as the sealing ring which comes stock on the exhaust of several
Astra F models.
However, we can supply the ring, if you have trouble in locating one
locally.
Also, please let us know which flex joint parts you need, beside of the
ring.
Do you need bolts & springs only, or the entire outlet/connecting pipe?
We will be more than glad to help you.
Best Regards
Alessandro Gilli
Supersprint
Italy'