Ok, I just visited Overstock.com and found they have some sail shades. One is triangular, one is rectangular.
Link is below to the first one.
They are not the heavy duty ones like those made by Sailshadeworld.com in Australia, but they might do the job.
It's worth a look anyway. Even this late in the season, our hot weather will extend for another two months and maybe a little more.
I got a coupon code in the mail which will take an extra 7% off the sale price, if ordered before August 29.
It is 151576 applied at checkout. Hope this helps?
Here is a link that might be useful: Sail Shades at Overstock.com