Expanding holes in sheet metal is easy. Tapping holes, welding captive nuts, and bending are a lot trickier.
Brass standoffs with a nut on the back are also readily available and cheap.
I was thinking more of moderate scale. I’d totally rivnut one SBC if I had a friend who could show me how. But I’m not doing twenty or small batches for resale. I think there’s a fraction of people in that demographic for whom a rivnut is still fancy tech. I’ve only heard about them from watching Bad Obsession Motorsports (with Rex Hamilton as Abraham Lincoln) and rivnuts are like a running gag argument between the two hosts. Welded captive nuts always win. Every time.
I just looked at a rivnut catalog, and couldn’t figure out how to find a nut that would work with standard computer case screws, or in fact what tool and nut combinations worked together. I’m not gonna sort that shit out on my own.