I know that having the lens attached is a bonus for quick removal for shooting straight away, but is it really necessary? I have decided to have the camera and lens separate in a smaller backpack and take the lens and camera out prior to shooting and then sling the combo over my shoulder rather than carry it in the backpack and then get it out. I find that the backpacks for the camera and the 600mm f4 TC lens attached is just getting way too large for comfortable use in most cases. Like @
Krümelkraft above, I do have the Mindshift Backlight 36L which just fits in with camera and lens attached but it is also a largish bag. I use the Mindshift Backlight 36L when I want to take another lens or two also as shown by @
Krümelkraft above. I do have a the Lowepro Lens Trekker 600AW but never use it as it is too large for me IMO.
If I am just using the 600TC, I now mainly use the Vanguard R48 which is very small and light but you have to have the lens separate from the camera. As I stated above, I just take the camera and lens out and attach them before walking to where I am shooting. I can then put the backpack on my back and the camera/lens slung over my shoulder. As with my preference, the Vanguard R48 and the Mindshift Backlight are all rear entry backpacks. Rear entry meaning that the opening to get to the gear is against your back and thus when you put the bag on the ground after taking off your back it is open upwards facing you. This also means that if you put it on a dirty or wet ground, you do not get that dirt on your back or get a wet back. In a pinch, these bags can be accessed with the belt still done up and removing the shoulder straps and then swiveling the bag around your hips and accessing the gear without putting it on the ground.