I have a trick I learned from my Uncle long ago. I take newspaper with me in the house and when I catch a northern, I wrap it in several pieces. I then put a rubber band around the nose and tail section. I lay the fish on the floor of the house where it remains cold, but won't freeze solid. The newpaper absorbs a lot of the slime and by securing it with rubber bands keeps the fish from coming out and keeps him oblong, instead of ending up 'C' shaped. It works too when placing them outside as well because the paper keeps them somewhat insulated.
Also, if northern is frozen solid, let it thaw in a sink with cold water. Within an hour it should be thawed and the cold water will keep the meat from getting soft.