In reply to Smelly Fox:
When I was shooting mountain landscapes and climbing/scrambling shots with a Hasselblad, I had a two-compartment climbing sack - the smaller, bottom compartment, intended for a sleeping bag I think, being unzippable from the outside. I then bought a solid block of quite hard closed cell foam plastic, cut to size to fit the compartment, and then cut holes in it for each of my three lenses, the teleconvertor, my spotmeter, and various filters. The holes were deliberately made to be quite a tight fit. It had a base of foam plastic glued at the back of it so that the holes did not go right through.
The top part of the sack was for spare clothing, food, water etc, and also for the camera, when walking in (though a lot of the time it would be round my neck in a padded case.)
The beauty of this method over e.g a custom made Lowe camera sack is that it just looked like any ordinary climbing sack - there was no hint that it contained a lot of extremely expensive camera gear (though a bit of a giveaway was that I had my Manfrotto tripod strapped to the bottom of the sack - but I guess most people assumed I was simply carrying an ordinary 35mm camera). It also of course had the advantage of being a climbing sack too - with crampon/axe straps etc.
In the field the system allowed an extremely rapid change of lenses, by simply unzipping that outer, bottom lid without having to rummage about in the main sack (always a highly unsatisfactory system.)
When I went on a shoot in the Everest region I had a porter who couldn't speak a word of English, but he was v bright. Within a few minutes he's got the hang of the different lenses, and when I wanted to change focal length he'd unzip the lens compartment, take out the lens and hand it to me, and I'd give him the previous lens to put away. All without even having to take the sack off my back. Sometimes, incredibly, he even anticipated what lens I would need next. I would turn round to ask him for the 150mm, say, and he'd already have it out, holding it in his hand.
BTW, the sack was very, very heavy by modern digital standards.