I have two tool tubes on the left-hand side behind the left pannier rail (will post pics when I remount them tomorrow... took them off as part of my servicing efforts).
One contains a 1l fuel bottle (just in case, you never know), the other contains (shockingly) tools!
I have a selection of every spanner, socket, alan/screw head and even a better spark plug wrench that the bike uses anywhere!
The idea is that there should be no circumstance in which I don't have the necessary tool with me so long as it doesn't require "the monster torque wrench", in which case I'm boned! It's too big to fit in the tube, and too expensive to leave anywhere else on the bike anyway!
There are some tools I carry that are slightly too big for the tube, but fit nicely under the saddle (in the "tool area").
Funnily enough I've had these tubes on there for 3 years, and they don't have locks on them! Never has anyone actually tried to open them... even when I park it in the centre of Manchester, or worse yet a Council Estate!