Nickle, medela sold, for a while, an insulated bag that came with a specially shaped ice block thing - you know, the hard plastic things filled with liquid that you freeze to keep things cold in an esky... anyway they make a special little bag, with a special shaped ice thing, that sits around four bottles. That's what I used - no risk of it being in the fridge, it was always at my feet, kept nice and cold by the ice block thing.

Its quite compact, fitted in my bag and I never even filled four bottles at work but it was useful.

Not sure if they are still selling it, but it might be worth looking into.