Do a search for' cast iron strap on boss with gasket ' they are expensive items £50 approx and you will also need a suitable size starett hss hole saw.
you'll need a new or very sharp 57mm holesaw and use it on low speed in the drill, it'll go through like a hot knife through butter!
although it says you don't need to, i always put a little silicone on the fitting where it buts up against the soil pipe. 'just in case'. also, make sure it's the right way up, (it has TOP printed on it )
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