Dave,
Since in reality, most of the spark plug is covered by the boot on the end of the lead, I just take a short length of insulated wire, strip out the wire, and insert my colored spark plug wire into that, and glue that to the block.
The plugs are too small to see very much of (in 1/24 scale).
Getting hex stock that small to turn down on a lathe would be hard. There may be someone who manufactures, or extrudes, styrene in hex shapes. Maybe Evergreen or Plastruct does. Check with them.
Alternatively, have a look at Grandt line bolts. They have lots of different sizes of bolts, and maybe they can replicate the hex portion of the plug.
They also advertise in FSM, and armour fanatics have been using them for years.
Cheers'
LeeTree