Two words: Think Tank.
I've spent years and hundreds of pounds trying to find the holy grail of camera bags that is easily accessible and durable enough to put up with the amount of crap I put them through (think weeks in the woods/mountains, climbing, etc).
I recently picked up a Think Tank Pro Speed belt and a holster and I've been VERY happy with the system, sits perfectly under my harness/pack straps without any issues and they also do a variety of chest/shoulder straps to boot. The holster feels practically bomb proof and has all the features you could ever wish/hope for (expands for use with telephoto lens, pockets/pouches for random things, rain cover, etc, etc!). Me likey!