Behind a panel in my house is the exposed end of a long-disused waste
pipe - quite likely there was once a sink there - which I suspect of
being the source of an occasional bad smell.

Presumably I should cap it, but as you can see from this photo it's
not a neat end:

http://www.cogsci.ed.ac.uk/~richard/pipe-end.jpg

The outside diameter is about 7cm.

There are flexible rubber caps for sale, e.g.

https://www.amazon.co.uk/Cloverleaf-...dp/B078HHNFHS/

but I doubt it could be attached securely. Would a piece of some kind
of flexible rubber sheet and a jubilee clip do the trick?

-- Richard