I recently noticed the flanges around some of my home's radiator pipes were missing.  I don't know if you can even buy these anymore, so I printed some replacements.

Printed with ASA with 0.2mm layers, “face-up” for a cleaner finish without post-processing, but you can save some filament by printing upside, and cleaning thing up after.

The pegs for joining the two halves are intentionally quite snug, but can be sanded down if need be.

uploaded in two sizes:

flange72 →  3" flange for 1.3" pipe 

flange65 → 2.5" for a 1" pipe

i can upload the (very simple) model if anyone wants to customize