Rain vent cover for pipe ducts. I have used mine for Kiln duct and it has been working well so far. There are two categories here:

1. one without screw, you can print and directly insert into a pipe duct. Feel free to change the infill density, since it can vary according to material and strenght you need. I designed it to have some tolerance so that it can be inserted into a pipe easily, but I still recommend doing a test print by cutting few bottom layers for a successful print.
2. The one with screw is you need to manually screw them into a surface that you require a duct, mostly when the pipe is not extruding out from the wall. The model is printed into 2 parts, first being a solid base for the screw to be held, while second as a cover.

I have used both models for myself, one for a laser cutter vent cover and the another for my kiln duct. Both worked great so far. Please let me know if you want different diameter of the design. I can add it for you.

I have added print profiles for both, with different diameters. Reference that for your prints.