Go to Alpkit.com and buy one of their drybag backpacks. They do a new commuting rucsack as well now. Better yet do your back a favour, get a saddlebag and let your clothes keep you dry/warm/sweat free like they were designed to.
@AlexB started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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Go to Alpkit.com and buy one of their drybag backpacks. They do a new commuting rucsack as well now.
Better yet do your back a favour, get a saddlebag and let your clothes keep you dry/warm/sweat free like they were designed to.