Your "extra weight" comment got me. I have been bike commuting for 23 years to stay in shape. I have never worried about bike or component weight since any extra weight would mean more of a work-out (althought maybe not so much) I use a backpack for commuting. I wear it on my road bike, and rack it on my other two bikes. I'm on my 4th backpack in 23-years. For me, toughness is the main voirtue I look for. If weight is not a concern, then the most robust bag within budget is the goal. I prefer wide, long straps that are solidly atatched to the bag. I wear the bag loose-ish and let the weight rest on the small of my back. This allows air to blow down my back. I like multiple compartments. I get water-resistant backpacks, and carry a trash bag to bag it when it rains. I can't cover the whole thing, but enough to prevent it getting soaked. Perhaps next time out I will see if I can find a waterproof bag. WHat I did a few years ago was go to Goodwill and other thrift shops and bought used bags of different styles to try out before I committed to a new bag.