A flange that I used on the exterior of our house to cover the gap between the pipe and the opening in the siding. It's made as two identical, interlocking pieces that fit around the existing pipe and secured with caulking or glue. It can be mounted with the collar out or in to fill the gap.

Fully customizable OpenSCAD file included, with the important parameters defined at the very top.

3D printed on a SoliDoodle out of ABS for durability.