In reply to J1234:
I've looked at the PacSafe ones (used to work in a gear shop) and they do a range of options, the biggest of which is a steel mesh drawstring bag with steel cable that you crank down over your pack/holdall/whatever and then use the padlock to both stop it coming loose and lock it to some immovable object.
It's a nice idea and will deter the casual thief, but they do weigh a ton, so not ideal if weight is a major factor. The size of the mesh holes means it may still be possible to get small items out if the material is slashed or pocket zips can be worked loose - you really need to have it as tight as possible around the bag, so choose the correct sized one.
I do have one of their money belts - that's quite a neat design with steel cables in the waist belt and a concealed buckle, so a thief can't sneakily undo/cut it and leg it with your wallet.