Thanks, Robert. I'd rather not deal with neck cords or zippers. Right now the Backrollers work alright, especially since I removed the internal pockets that would catch bags and items as I shoved them down, and I will investigate the North St. bag.
Funny to admit, but if my criteria were purely practical, I'd go back to the very best ever grocery panniers I've ever used, no exception: a pair of $4 (on sale) 13 gallon plastic trash cans from Target, strapped using dowels for support to the rear rack. huge capacity, no obstruction at all for shoving things, bags or otherwise, in, and plenty of room in back for reflective stickers. Their fatal downside, I have to admit, was that they made me look like a dork.
Even funnier: I recall how, back 10 years ago or so when I was young and fit and green -- mid 50s -- I was spinning along on that 1973 Motobecane making time in the 67" gear with these trashcans attached, and was unintentionally gaining space on a CF lycra clad -- I will use the word "roadie" because he got annoyed and sped up to pass me; so the derogation is appropriate. He then missed the turn I made, and came back to pass me again. It didn't click with me until I got home -- my reaction at the moment was, "what's this guy doing here again; didn't I just see him?"; if I'd understood, I'd have run him off the road. But I am sure that he got upset because what to his provincial eyes looked like a fredly dork on a beater bike -- and with a long pony tail, no helmet, and in more or less civilian clothes -- was disrespecting him.
Actually sold them with an old beater bike to my NDN who used the setup for some years, though for the last 7 or so bike and trashcans have been slowly disintegrating in the NM sun.