We've had allot of that threatened lately... We invested in a blanket... It has 4 clips and straps which actually secures it quite well :) The car looked like a weather balloon with it on. Can only imagine what people thought when they saw it. Hehe
Have not tested it in hail, as we've not had any of the threats come through (Thank God) but it faired very well in the rain!
A word of advice? Have someone help you take it off, preferably in calm conditions. I nearly blew away with it trying to take it off by myself... Hehe