The blocking diode is normally shorted by one pole of a changeover switch. As you throw the switch the diode carries the current in the short interval before the generator side of the main rectifier is shorted. It is better than momentarily open circuiting the generator as Rich proposes, but that would no doubt work but would need a better switch.
The diode could well be a single phase bridge. You could use the positive as cathode and for the anode you could join the two ac leads together. It wouldn't matter if you used the pos and neg leads and ignored the ac ones. 2 diodes in series wouldn't matter.
Alternatively you could use a single diode with a rating of at least 20A. The normal ratings are for where the diode is always in circuit. In this case it is in circuit for only a fraction of a second and even a 20A diode will have an ac half cycle surge rating of about 200A and your time interval should be less.
A diode extracted from a car alternator would be ideal, you could parallel the 3 diodes of a car alternator half bridge.
Flux