If you've got some money to blow, you could look into building a "solar tracker" that moves with the sunlight and will automatically cover your plants in the afternoon.

Here's somebody's homemade setup that looks like it could be adapted for your purposes, the most expensive part will be a solar panel capable of delivering enough power:
My Homemade Solar Tracker

But instead of following the sun you'd want it to slide a large box (like a waterproofed refrigerator box) over your plant at the same time each day.