I commute on my Dyna just about every day and have found the HD sissy bar backpack to be the most practical. I can keep all I need in it, such as rain gear and chaps rolled up, plus my lunch and laptop. Easy to take on and off and has plenty of room of other smaller items.

http://www.harley-davidson.com/store/sissy-bar-backpack

 

Walmart bag, $35 and fits a ton of ****. slide right over the sissybar and has quick connects. Fits my laptop, lunch, two water bottles, and a hoodie.

 

Goodness, Harley is proud of that backpack at $150