I needed an endcap to close up a wall through that was no longer needed. And since the home depot was closed already, I simply printed one to close up the pipe with an ID 95 mm/OD 100 mm. With one to two wraps of ductape, this seals the pipe to cut off any airstream.

I included a small 0.4 mm recess for the pipe to sit in, so the air seal would be a little better without additional sealant, but one could also use it to carry additional sealant.