Hi
You should also consider building a free pendulum clock reference.
If I ever get a supply of circular tuits, then that's something I have
long considered making... Even a single-pendulum (non-free) synchronome
would be a fun thing to build
I've considered adding one to my grandfather
clock.
These were used as references until the quartz oven references
replaced them.
I am told that a free paenddulum in a vacuum chamber (to remove the
errors due to air resistance, often called 'barometric errors', since the
air resistance depends on the barometric pressure) actually has a better
long-term stability than most quartz crystals.
-tony