Make the investment, buy it and bring the shade with you.
http://www.ezup.com/products/pro/hub.shtml or even 1-2 of these
http://www.ezup.com/products/pro/eclipse2.shtml in 10x15 (no center pole lke on 10x20). You can find some of the "cheaper" ones on ebay also which work pretty well just not as sturdy. Don't forget the weights for the legs to hold it down in case of wind.
I have 2 cheapo 10x10 ones that get used for every party and are awesome. I have used it for the car but it's just not quite big enough which is why i suggest the 10x15 or better 16x16. Also seen some guys use a big tarp attached to the side of their van with heavy duty tent poles staked down but that depends where you are and if you can stake into something safely without damaging property.
I only waxed in the sun a couple times and it sucked no matter what wax I used.