Thanks to comments from Doc Pedersen and nsayer, I ordered an Electrical Frequency Controlled (EFC) Ovenized Crystal Oscillator (OCXO) with TTL output to improve the short term stability.

It is a muRata OC2545-DT 10.00 MHz OCXO Crystal Oscillator.

OC2545-DT Datasheet

Thanks to the commenters I now have some new parts on hand:

muRata OC2545-DT 10.00 MHz OCXO Crystal Oscillator

12 bit i2c DAC board (To generate the control voltage – Delta Sigma w/Filter)

Adafruit Si5351 Clock Generator Breakout (To generate microcontroller clock)

RS232 to TTL converter board

For more detail: Rubidium Disciplined Atomic Clock