I carry my EF 500 F/4L IS in a Lowe Pro Flipside 300, complete with both 1.4x and 2x extenders - that goes on my back. I carry my 1D MkIV separately, probably with a near standard or 70-200 F/4L zoom on it. I trundle my Benbo One tripod in a wheeled trolley.
I bought the larger Flipside 400 thinking that I'd be able to carry more on my back, but it didn't work out. My copy wouldn't take the EF 500 F/4 L due to it having thicker and stiffer padding! Weird.
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Milton Keynes, UK