[Christ]

Yeah, they're called Coffee Urns, ironically. 2 gallons is more than your original coolant system held, so it's probably sufficient.

What are you using to bleed heat from the e-motor you have?

If you're already bleeding heat from there, you could always duct it back through the cabin air filter housing and have semi-free heat, since it's heat that's already being generated.

If you're not bleeding that heat off the motor, maybe a motor cooling system is in order? You could use the circulation of your coffee urn cooling system to help cool the motor too, I would say, and that would add a little extra heat to the coolant, making it warmer for the occupants.

Food for thought.