I've replaced the internal crystal oscillator and temperature sensor with an external oscillator and analog temperature sensor.  This provides both better precision for temperature measurements as well as tighter thermal coupling between the oscillator and the temperature sensor.

• Oscillator: ASE-25.000MHZ-L-C-T
• Temperature: MAX6577ZUT+T

The frequency output of the MAX6577 provides better than 0.01 C precision when combined with 32-bit timer capture.  This allows for a PPS RMS tracking error of less than 10 ns.