Summary

I have my roof covered with pebbles, and for a long time they have been slowly tricking through the main drainage pipe because there's nothing stopping them. I tried one of those generic ones that you can find in hardware stores, but since they are flexible I still had problems with pebbles occasionally coming through. My solution was to 3D print this one. Due to its conic shape it should fit any tube between 60 and 100mm diameter.

It will be exposed to the elements, so I strongly recommend using a material that can sustain high temperatures (I used ABS).