what's the best way to fit an inspection / rodding hatch to a cast
iron downpipe?

I can cut the pipe with an angle grinder, but how do I attach and seal
a hatch?
it's a 3 inch diameter cast iron waste water & rain gutter pipe in the
twin stack system of a three story tenement flat.

is there something I can use in PVC / plastic ?

how about achieving a permanent waterproof seal ?

TIA


-A-