I got a rubbermaid under bed storage box. Its a great size don't know exact dimensions, but its thin, wide and long, perfect for holding fish. I slide it under my rowers seat.

I also keep my fish in a burlap sack that I keep wet. If its warm out I'll also bring some ice in a six pack cooler and throw it in the sack with the fish. The ice will usually last all day in the wet sack.

I got my box at Fred Meyer.