[Gandalf1953]

If you want an individual bag just for the lens when needed, I got one of these especially for the sigma 300mm f2.8 including hood reversed and it fits a treat.
 https://www.amazon.com/Ape-Case-ACPROLC1...Bcase&th=1
 According to the spec its a little small for yours but of course the 17cm is the diameter and there is give in the sides. 

Main thing if you go for single lens bag get a good strap.    I got this one and its brilliant as it is significantly more non slip than anything I ever tried, on the same side as the bag, or across the chest. https://i-stayput.com/pages/the-i-stay-n...-bag-strap

 I've even replaced think tank straps with one :   It seems to me lens weight increases exponentially with each mile you walk! - which is why I've now P/x'd the  lens for the Canon 100-400 mkII

Thank you Dave, but I think I will need the back pack rather than a lens specific case as I will have other gear too.
The Canon 100-400 mkII is a great lens and really versatile, I love mine!