Bag for carrying DSLR body and lens while climbing/hiking inside backpack

I carry a 450D with the 70-300mm L IS USM attached and a 17-55mm and 1.4x converter next to it in the camera pouch.  I then carry the 300mm L in a protective drawstring lens bag down behind the camera pouch part accessed from the top of the bag, so it uses most of the length of the bag.  This still leaves room for a jacket, lunch, binos etc.