I would say cheat.
- Get a drinking straw of like size.
- Flare one end of the straw to match the bell shape.
- Cut a V shaped section from the side out of it but make sure there is enough straw left to retain it's shape and not splay open.
- Fold the straw at the cut apex and glue with crazy glue.
I think this would only work with VERY small scaled vents.
The narrower the V cuts and the more you have to complete the 90* angle the smoother the curve.
I'm not a ship modeller but it's how I would do it. Plastic straw could be substituted for paper if you create the cylinder from paper first.