I my experience, "professional" pre-made boxes have been "silly expensive" for at least 40 years. Unless you find one of the right size on the surplus market (increasingly rare, as custom-made boxes have become more popular for commercial gear, and surplus stores have been done in by cheap overseas "new" components.)
In high school, I used to build projects into cardboard boxes, and after graduating from college I noticed that your average "home-supply store" looked like they had a better selection of interesting and not-too-expensive boxes than what you'd get from an electronics supplier.
(of course, 40y ago, "silly expensive" was a much lower absolute number than it is now.)