Maybe you could consider 3D-printing a mold, to make your chimney from
plain common silicone caulk squeezed into that mold?
I don't know whether silicone caulk adheres to ABS or the other plastics
you can 3D-print. But I have made many special silicone parts by making
polyethylene molds on my lathe, and filling them. Silicone caulk doesn't
adhere to polyethylene, so no separating agent is needed.
The molds need to be fully open on one side, so that the silicone caulk
I have also made short sections of silicone hose, in special sizes, by
extruding the silicone through nozzles purpose-made on my lathe. It's
best to let them hang vertically down from the nozzle while they dry.
There are also two-part silicone compounds, which dry without needing
air. If you can get them (I can't!), you can use molds that fully
enclose the piece, producing a more precise and better looking silicone
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