This is a clip on cover for a greenhouse gutter pipe to prevent leaves and other stuff washed off the roof from ending up in the attached water butt.

The gutter it attaches to here is 34mm outside diameter (2mm thickness) and is the official Elite greenhouse rainwater kit.

Tinkercad model can be found here to modify to any different pipe sizes - https://www.tinkercad.com/things/bWLcgPr2iP4-horizontal-small-gutter-filter