Have you mentioned what color your car is, including the interior? White or silver, with light interior, will stand the best chance of minimal heating, while black would do worst, obviously. I was going to suggest a thin, white car cover that goes on and off easily, but that might actually trap heat rising from the pavement. A solar window ventilator might help a bit.
2013 "Brilliant Silver" SV with Premium and no QC, a 2009 Vectrix VX-1 W/18 Leaf modules, and 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.