Anything that will carry a current without involving you in the process is adequate. I actually carry in my glove compartment a short piece of wire with the insulation stripped off the ends for just this purpose. Occasionally my ignition switch acts up and fails to engage the starter but it hasn't become consistent enough for me to actually replace it yet so the wire is just insurance.
Don't forget to glow your plugs before doing it if you want the engine to start. Also, make sure the transmission is in Park because you are bypassing the neutral safety switch.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13