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Bank bags made of canvas used by businesses to transport lage coin orders make excellent anchor bags.  Many retail stores or restuarants probably have them lying around and they'd be free for the asking.

While your at it, get a few extra ones for cooling your fillet fish during hot weather or when you catch dinner early and have a few miles to paddle/portage before setting camp.

Fillet your catch, seal in zip lock and put in canvas bag secured with Bungee cord.
Soak canvas and store under seat out of direct sun. Wet as needed. Will keep fillet cool/fresh for hours even on the hottest day.