What flyback diode should I use for my spark gap transmitter?

The power is a 12V car battery ( I think 20A) and I have a car ignition coil with an output of 45kV.  I want to use a 555 for the interrupter and wondering how to size the flyback diode to protect the circuit.  I have limited knowledge of circuitry and trying not to electrocute myself!thanks!