I've researched these and am tempted to replace my in floor fish box with one. The best one seems to be the Reliable Fish Kill Bag which has a built in drain spout. Pricey though.

http://www.sportco.com/productcart/pc/RELIABLE-FISHING-PRODUCTS-KILL-BAG-28-X-48-466p52875.htm
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"You learn more from losing than you do from winning." Lou Pinella