Well, the air-conditioner cover I made in the fall lasted its first winter ... if winter'll ever end. It only blew off once before I put some bricks around it (was too lazy to make a drawstring). It's made of Top Gun Lite (aka Top Gun 9, aka Mustang?), an acrylic-coated polyester fabric designed for boat covers -- which of course makes it perfect for covering your A/C unit. :)
Rochford Supply, up in Minnesota, has roll ends (1-3 yards) for cheap.