Tell ya what works on my 300/4.0USM
I've got a length of camo cloth - tiger stripe camo to be exact - got it sewn into a "tube" and elastic put in both ends. Done. Works great.
I've also got a "green ghost" woodland camo personal blind, with extension for as long a lens as you might want, also with elastic on the end. Its open in back (thank the breeze Gods!) and other than being hotter than two rats humpin' in a wool sock, is a great personal blind. It fits head to toe, even while on a chair, sitting behind a tripod.... a nice personal piece of cover.... and it works (for the most part).