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