That is pretty neat. The string will also keep the shade cloth from blowing away!I'm leaning toward something like this with hail netting instead of shade cloth. I did this last year with plastic sheeting and it worked great as a row cover to extend the season, it protected my starts from a late snow, kept transplants from being sunburnt, and warmed the soil quicker. But it didn't work past that because it held the heat too much and didn't allow water to come in. I love the string over these. It allows you to just push or pull the fabric up or down to exactly where you want it.
I will have to file that away for future ideas for my garden.
If you do that with plastic, make sure you can open the ends. That way you can let the heat out.
I agree about the hail, you just never know when or where it will hit.