I have the Lowepro Toploader AW Holster Bag, but it wont fit my 28-300L with the lens hood on. It'll fit the 1D with the big lens, as long as I take off the hood, but it still bugs we.
That lens is as long as your 100-400, so I'm sure it'd fit. It's a great bag and I beat the tar out of it and it takes the abuse. For the price, you really cant beat it. I tend to go through a lot of bags, as I leave them in the bottom of my kayak while paddling, strap them to my chest while riding the motorcycle, or to my back while mounting biking or hiking, and this is one of my favorites for the light traveler.
Good luck and here is a link to the bag
http://www.bhphotovideo.com/c/product/9 ... lster.html