QuiltE

Make it in your crockpot/slowcooker at home.
Easy to transport and no need to reheat.
Depending on timing, you may not even need to plug it in when you get to the event.

Lots of options to choose that will meet your other requirements.

One of my lfavourites is to take either scalloped potatoes or garlic mashed a la crockpot.
Most people will never take potatoes.
Bonus ... most people devour either of the above ... and you will be a hero!