TL/DR: How does someone make a good horn antenna system without using expensively made parts? Welding thin steel for this is not something even pro welders can do (it's a specialty app.) What are DIY options then for joining sheet metal. What is the most careless thing that could still function, even fn well (like 300 sheet metal screws poking inside.)
Beware, this is long: I don't expect to match the efficiency of radar components. I made one out of copper/solder about 20 years ago using 2 pieces of copper and it had a short waveguide section for a probe or loop antenna. It was maybe 22" aperture, I remember getting all the dimensions using a C++ program, drawing the piece in autocad and hijacking the plotting machine to print it out at work lol (it wound up being far from the printed accuracy) ... Copper has skyrocketed in cost since then and it's heavy and deforms easily. ... On the other hand, thin steel requires an exceptional welder I don't even know where to find, and have asked all around before. For very narrow gauge, as in the sheet metal used for the waveguide in a microwave oven I think it's a process involving a roller disc inside the waveguide making spot welds at the closest interval possible and doing multiple passes until it's continuous.
Literature(s) about horn antennas and waveguides just say "smooth" and "continuous", no dings or warpage either. Not what a little bit of that might do. What for example would a rivet's profile do to the inside of an otherwise mirrorlike inside wall with exact angles. Not that getting it right the 1st attempt is ever expected, but I seek thoughts and advice from others before wanting to invest my time and resources into a project. I've seen an example of someone making a horns joining a corner with adhesive backed foil tape (?! they had no seeming concern about this nor data on performance, Maybe they even knew what they were doing I just doubt it.) I don't have any good examples for reference or really any examples of similar steel horn antennae. I think of all the such things involved with how the steel overlaps and maybe using a brazing technique (no familiarity with brazing) or something like closely drilled pairs of holes with twisted wire clamping edges/faces together. L shaped edges and the like create a problem inside with a valley/furrow that could be mitigated maybe a little by filling it with conductive epoxy or solder (solder in reality would be hard to do, it running all over the place or being lumpy.) I've never seen a homemade antenna that was any good besides ones made from copper, and those 2 were smaller than the one I did.