I'm a firm believer in the rock salt theory (no pun intended). I use 1 cup per quart of water, usually the quart is half ice/half water. Add 1 tsp of alum and one bottle of anise oil extract.

I usually do about 12 dozen at a time and put them in freezer bags 2 doz/bag. Then put these in the freezer. The rock salt doesn't allow it to freeze and keeping it very cold keeps it real firm.

Between me and my wife we put 46 fish in the boat last year and that's all we used the entire season. I think it works.

And like Downriggin mentioned, this will last for 2 months or more. I brined some up in early July last year and was using it in Sept. for silvers in Sekiu.