Patagonia boat bag in near mint condition. I purchased new, used for one trip, then went a different direction.
It has a formed "bath tub" bottom and a collapsible top, carry handles and a shoulder strap. Typical Patagonia bombproof construction to last a lifetime.
Dimensions are approx 23"L x 15"W x 10"H (height can be compressed lower if desired).
Waterproof bottom, top and zipper. The sides are what I'd consider "splash proof".
$100 obo shipped CONUS
free pickup in Portland