ignition switch

Simple – stop using the key switch (no ignition on a diesel) to operate high-load circuits like the starter solenoid and glow plugs.  Go get a couple of 30 or 40 amp 12vdc relays and some 10 gauge primary wire form the auto parts shop and rewire your starter solenoid so that the key switch only activates the coil on the relay, which then transfers power from the battery to the solenoid by the 10 gauge wire.  The wire that currently runs from the key switch to the solenoid is what you use to energize the relay coil.  I mounted my relay right on the solenoid for convenience and minimum use of wire, but it can be mounted anywhere convenient to you.  Do the same thing with the glow plugs. 

If you have additional high-load stuff like auxiliary lighting, I recommend switching them through relays as well.  Most panel switches are only good for 10 amps, which isn't much when it comes to bright lighting (unless you're using LEDs).