One practical warning: put the egg in gently and consider not using your best flask.
Dropping an egg into a narrow glass-lined flask is a good way to chip the liner, and a flask with a cracked liner loses its insulation permanently. Lower it in on a spoon, or use a steel-lined flask for this.
Also, the egg may crack from thermal shock if it goes straight from fridge to boiling water, which brings you back to warming it first.