I've been wanting to build a Raspberry Pi controlled Nixie clock - but instead of a Atomic Clock, I'm going just use a USB GPS dongle. Also set it up as a NTP server for machines on my home network.
I'm settling for a regular RTC[2] rather than an atomic clock for the fallback though.
Designing the PCB's is taking a bit of time though, me being rather new and Nixie tubes being 170V does slow things down a bit.
[1]: https://news.ycombinator.com/item?id=28372861
[2]: https://www.nxp.com/products/peripherals-and-logic/signal-ch...
Mine was built with a much older version of this board - https://store.uputronics.com/index.php?route=product/product.... I like using the HAT as a dedicated solution instead of the USB route.