Agave Winter Protection

I decided to start protecting my Agave Parryi and my Agave Harvardia plants 3 weeks ago by putting some old glass window panes over some posts and concrete blocks and other junk I found lying around to protect them. I put clear 4 mil clear window plastic over the windows and covered some of the sides with it to keep the moisture out. I left some areas open on the sides to help circulate the air. It is best to cover Agave's early I am told because this way the soil/sand around them can dry out, as they tend to rot when they are both cold and wet, its not the cold that gets them its the moisture. The dirt they are planted in is mostly corse sand and small rocks with some potting mix mixed in. It seems to drain pretty fast so that should help also to prevent root rot. They have to be kept dry all winter long. We have very wet winters here in southern Illinois USDA zone 6b, so they would not survive the winter here without moisture protection.