I have been fortunate to be able to keep extra clothes at the office. I have shoes, a jacket and a winter parka there. If that is not possible, I would look into a basket in the front, or rear, or Wald folding baskets, or paniers. But to keep it simple, find a good way to roll your parka and bungie it or strap it with velcro. Then bungie or strap it to the outside of whatever you're using now. Perhaps you could find a small waterproof bag or duffell and strap that to what you're using now.