I appreciate the ideas, y'all. I took the first baby step today by stashing 40 gallons of water in and around the garage. This is a similar technique to what greenhouses use to regulate temperature. The attraction of this technique, for me, is that it's extremely cheap and easy. I may do a bit of research to see if 40 gallons is reasonable for a room this size.
While I was up in the attic, I saw that the garage side is not insulated at all. So I'm off to Home Depot to fill the Leaf up with insulation and take care of that. No wonder it got so hot in there.
Btw, lest you think I'm crazy with talk of phase change materials, it's something that every mechanical engineer and chem major learns. Maybe even high school chem students. Tesla's Gigafactory will have a "thermal energy storage tank" that may use the same concept.
https://electrek.co/2017/02/07/tesla...ry-production/