[bav]

Put a solar shade in the front, cover the other windows, snooze away. But that's gonna be very unpleasant. It's not super hot yet, being only June 1, but as I type this at 00:20 in Vegas it's 87F out there. Probably a bit hotter in the middle of a Rio parking lot radiating stored heat from the concrete. There are NEVER 8 pleasantly cool hours a day in mid summer to make sleeping at all comfy. If you were out where a breeze could hit you it might be bearable. But in a car? I suppose if you're the Navy Seal type who's been taught to enjoy sleeping in 100 degrees, ok. Otherwise I think you'll be awake in the heat until 3am, sleep for 3 hours, and then be awakened by the heat about 6am.

Your plan works for the "gonna be in Vegas one night, do I have to get a hotel room" trip where you won't die if you only managed to eek out 3 hours of sleep. Probably not a good long-term stragedy.