The problem with building these things from scratch is component cost - 48 pots isn’t cheap, and the size of a PCB to mount them on makes it a moderately expensive board in low volume. Plus you’d need some kind of multiplexing. It gets to the point where a secondhand one seems even more of a bargain.
(Every now and then I really think about designing/building a Pocket Dial for the modern era with USB/DIN/stereo jack. Maybe even Kickstarting the damn thing. Then I remember how much I hate the idea of Kickstarting anything)