MX5 parts Stainless backbox on a Mk2.5

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: __2.5
  2. I’m based near: __
  3. I’m looking for technical help or recommendations on: __Exhaust gaskets

Hi all, ordered a stainless steel exhaust backbox (No centre pipe) for my 2004 car as well as a gasket from autolink as mx5parts don’t have any in stock. However, looking at the gasket its diameter is smaller than that of the bore of the exhaust pipe. Anyone know where I can get one that fits? Or, being a crush gasket, will it expand when the backbox is bolted to the centre pipe? I haven’t the slightest how they work. Thanks for the help.