I've had no problems using an Ortlieb large saddle bag with the holder set to keep it at an angle that avoids wheel rub. I use some old innertube rubber to space it back off the seatpost. It's been large enough for 400k rides in bad weather and very durable and waterproof.
Second good option is the Arkel Randonneur rack and bag. The rack looks like a regular seatpost rack, but it's not - it's supported by the saddle rails and places no strain on the seatpost. I've also used that for brevets and it's worked well. The Arkel has a waterproof cover which comes with it.
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