@cypnk So far CSACs have been a bit on the large side for watches, someone did make one though; https://www.hodinkee.com/articles/introducing-the-bathys-cesium-133-atomic-clock-watch
And yeah, making your own movement has some seriously daunting precision requirements for a hobbyist
If you want to play about though you could order all the parts and assemble it yourself
@cypnk Seiko movements might be the easiest option for DIY projects, availability is good, different dials/handsets/etc. are easy to find, and they're relatively inexpensive
The NH35A movement for example is pretty damn good and you can get it for under $40
If you want to go upscale though you can find Sellita and ETA movements without too much trouble
@troubleMoney Holy Giant Watch, Batman! Yeah, that's a wee bit outside my comfort zone for size, but still pretty impressive
That assembly seems to be getting popular though. I've seen watch movement parts being sold online and a few movements even show up on AliExpress from time-to-time. If they're being mass-produced, then the biggest issues may be cost and space (also dust)