I don't have the 400mm f5.6
(wish I did though) but I bought my camo lens cover from Wildlife Watching Supplies -
Link.
They make a very decent product - neoprene with
Realtree® designs, and pretty cheap too - there's no button access though....you have to slide your finger underneath the neoprene to get to them - not a major issue though.
Link to Canon Lens Covers
My lens cover quite possibly saved my lens six months ago or so....my cheap shoulder strap broke and my 7DII+Sigma150-600 hit the floor!!
It was a heart stopping moment, but thankfully the lens hood had taken the brunt of it
(it was unbroken) - and the cushioning from the lens cover certainly helped.
I checked the lens calibration and I think I got away with it....suffice to say that I will not leave the house without the lens cover....oh, and I bought a proper, rated strap too!!