One option that I explored was using an inverter with two 200 watt ceramic heaters plugged into it, both in the driver's footwell. It actually worked ok, by keeping the driver's legs and feet warm with the cabin heater off. Two similar 400-450 watt heaters would work even better, and the same could be done on the passenger side, with all of them switchable. This would require a good, preferably AGM, 12 volt battery with substantial capacity. Before you do anything, though, I suggest you invest in the HEAT OFF switch developed for the 'water heater' Leafs.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.