Efficient Alternative to Backpack Carry

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Do you wear this back pack when you're off the bike, as part of your job or because you have a long walk from the parking lot? If so, I'd figure out how to strap it to the seat, as Cormanus suggested. It's the cheapest alternative and works just fine. A cargo net or bungees (look at rok straps) are easy purchases and can be used to secure other items. If you don't really need to use the backpack off the bike, then a solution like JPT suggested would be great. I think REI sells a dry bag that top loads but has an over the shoulder carrying strap. Pretty easy to strap something like that to the rear seat, too. My wife commuted to work on her CTX for a year using a backpack as a tail bag. Hope this helps. Good luck