Thanks so much for your response. I did some preliminary research before I posted last week and I like the Ibera top mount bag. I didn't consider that it might be tough to swing my leg over the bag when getting on and off the bike. Good point.
My main question is about the rack, rather than the bag. I would like to find a rack that fits the bike and then I'll consider whether I want a top mount bag or panniers. Do most or all standard racks fit onto the bike or do I have to go with the Sondors rack (and bag)?