This video compares various 3D printed plastic horns by looking at their radiation patterns and VSWR. Also shown is a simple experiment where we attempt to compare a few different solid materials placed inside of a waveguide.
EEVBLOG thread, Experimenting with waveguides using the LiteVNA
https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/
It was mentioned that The Signal Path channel made a video on 3D printed waveguides. The attached link takes you to what I believe they were referring to. He made a few horns and a filter. He's experimenting at much higher frequencies than I am and surface finish.
https://youtu.be/bo8pQmKgwgw?t=1966
During this video, I show a couple of printed horns from the following link. Maybe we can test them out in the future.
https://antennatestlab.com/3dprinting
Introduction to Waveguides using the LiteVNA, Part1
https://www.youtube.com/watch?v=h-bxyxLQtIk
Introduction to Waveguides using the LiteVNA, Part2
https://www.youtube.com/watch?v=qxH-a0cCY3U