4-5 feet of 18 gauge zip cord should about do it, that's 8 feet at .021 ohms/ft, .168 ohms, figure 3 amps for a dry glow plug, so that's .504 volts.
As mentioned, a sufficiently beefy silicon diode like a 10A10 and a short cable would also work. Might get away with a 1N5408 given it is only connected for 10 seconds/start.
Brett