I have a few bags, the ortlieb large saddle bag is the best I’ve used. lightweight, but more importantly, WATERPROOF, and airtight. rolls up small for the amount of stuff in there, and the best part, it unclips and goes with you easily. I have other larger bags, but this one can hold 2-3 tubes, all accessories, fold up raincoat, extra bars and foods. and if you have a few bikes, you purchase the mounting clips that stay on the seat rail separately. check them out. typ. about 35-50.00.
waterproof is really important. maybe not on ragbrai, but other rides and in the rain. it has a rubber interior. very lightweight, ripstop and durable. g