Have any others used a diode like the MC78M05CDTG
That's not a diode...
You can use a series diode but you'll get a voltage drop (about 0.7V) so the diode should be on the 12V side of the regulator. Or, a Schottky diode will only drop about 0.3V which you might be able to live with on the 5V side of the regulator.
You can use a reverse diode to ground, which will "short out" any negative voltage. In that circuit there should be a fuse so the fuse blows before you fry the diode & power supply.
and in case I'm an idoit.
Use a polarized connector and check everything with a multimeter (or LED voltage tester, etc.) before connecting the 1st time. Or, go-ahead and try a series diode on the 5V side until everything is working OK. It should "work OK" with the slight voltage drop.