Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive. https://www.microchip.com/en-us/product/csac-sa65 https:

Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive. https://www.microchip.com/en-us/product/csac-sa65 https://www.microchip.com/en-us/product/csac-sa65