I was looking around for a lock myself and found that most people dislike the retractable pocket sized locks because they just stop working eventually, either the retractor gives out or the lock jams and will not open. Also the plastic casing is just cheap and can be forced open if one was willing enough.
I just got this in the mail last week:
OnGuard Combo Lock Terrier Combo | SnowShack.com
Its convenient because its designed to be used with your gloves on. Also it does not have any fragile parts, its all just a simple bar/chamber interlock system so I don't imagine it will ever stop working or break due to the cold. Its not as portable as the retractable locks, but it still can be fit in an empty jacket pocket; I just recommend leaving it locked on a post if you only intend to dock at one location during your day.