You need several inches of "flap" when you are done loading them to roll the closure. Look at one in a store and throw some stuff in it to see how much of the top needs to be rolled to close it. You will find that your "grocery bag" is only about two thirds full when you can't put more stuff in it and still close it. That said, I still prefer my rollers over most others.

Most people do not strap stuff on top of the front rollers, but I more often than not have stuff strapped on top of each front roller. See photo, on one side I have a mesh bag that has a bit of damp laundry that I did not want to store inside the waterproof and air tight panniers, on the other side I have my rain gear strapped on top. I use the Ortlieb straps that go over the top. Being on front, I can see this stuff while riding so I am comfortable in knowing it is not going to fall off. This way some of the extras do not need to go into pockets.

And, handlebar bags are great for small stuff to keep at hand.



Indyfabz (above) suggested cloth tape on the rack. Thanks, I had not thought of that, I suspect that works well. I think I still have part of an old roll of cloth handlebar tape somewhere. I have used small pieces of inner tube rubber under a few layers of electrical tape, but that does not last very long.