I am currently using a Revelate Viscacha bag,
https://www.revelatedesigns.com/inde...alog/Seat-Bags
on the rear of my Honey Winterando bike. It fits almost as much as my Carradice Lowsaddle longflap did which was mounted to my Litespeed Arenberg on a Nitto Mark's rack with p-clips.
Positives:
Much lighter as no rack and Carradice bags are heavy. Bag does not have the wag the tail effect that the Carradice bag did and you can roll down tightly if not fully packed.
Negatives: Cannot see everything in the bag at once and must pack least used/heavier items closer to the seatpost. No side pockets for small items you may want to grab quickly.
I am not sure how waterproof the bag is as I have not ridden in extended downpours with it. You could always put clothes in a dry bag or plastic bags.
I used this bag for a full series this year with a
Sprocket top pouch
I would consider a small front bag for arm/leg warmers and gels/bars,sunscreen, rain jacket.