This is how I do it, I found a small laptop bag at a thrift store for two dollars and lashed it down with a couple of leather straps (milsurp I had laying around) with a little (probably unnecessary) assistance from one of my bungee nets:



The other leather strap is hidden by the bag behind where my Road Morph pump is poking out of the top of the outer pocket.

My girlfriend is no fan of used things and often raises her eyebrows to some of my solutions to things, but in this case she actually expressed that she was envious of this one and that it looked like it had been specifically designed for what I was using it for! I love that my lock makes no noise and is quick and easy to stow now (it's in the zipper pocket along the top). Of course when I need to run two panniers I need to pull this off, but that takes just a minute to unbuckle the two straps and I slip the lock (& pump & bungees) into the back of an Ortlieb.

I stow my bungees and my other bungee net in the outer pocket (under the webbing), and when I go on tour, I remount this bag outboard my starboard side bucket. It also gives me another accessible pocket.

Most days I run this, and an Ortlieb on the other side with my work clothes, toiletry bag, and lunch in it.