- A bikepacking-style seat bag, e.g. what Revelate Designs offers (https://www.revelatedesigns.com/index.cfm?fuseaction=store.catalog&CategoryID=4&ProductID=1).
- The more touring-oriented approach of a rear rack, with panniers and/or strapping a bag on top of the rack.
This idea looked worth trying so I pulled the trigger a while back, got it installed this weekend and got some good mileage on it to try it out. Compared to a seat bag, it is about as light, doesn't add width, but with a good dry bag it is completely waterproof and you have the potential for more capacity (I'm using a 15 L bag, whereas the large Revelate Designs seat bag is about 12 L). Need less capacity? Just use a smaller dry bag. Also I've been toying with the idea of trying a ThudBuster seat post, with this approach I don't have to worry about a seat bag being compatible with various posts.
Here's my Fargo with the rack installed:
And with a 15 L dry bag stuffed with random crap:
Pretty cool! I put the loop of the bag closure around the rack so in case it jostles off I won't run the chance of not hearing it and leaving it miles behind. Only problem is I have to figure out another place to attach a tail light. Maybe to a seat stay?