Normally, I like to use sling bags that go diagonally around the torso. However, for a long and heavy lens, it is mostly necessary to get a proper backpack. I also wanted a bag that fitted the Sigma 150-600mm lens with the hood extended, ready for use, and with the camera mounted. That means the internal section needs to be almost 0.5 meters long, not leaving many choices.
But one bag which is up for this is the Think Tank Glass Limo. The inlay picture shows how the bag is supposed to be used: It can take the Canon 600mm f/4 without a camera, or the Canon 500mm f/4 with a camera mounted: