I am away from my copy of "Joy of Cooking" , but the "recipe is in there. Just mix some non iodized salt with cold water (the proportion is in "joy') - and add the mussels and a handfull of cornmeal. I try to always treat my clams and mussels this way be4 cooking - it cleans them out and the difference is noticeable. Keep the mixture cool - they will easily stay till tomorrow nite. Sorry to only give u 1/2 an answer but some1 will have the proportion. ()the same as seawater , obviously)
Best Way: put them in a plastic collender (dry) or a mesh bag and put the collender (or bag) inside a larger plastic container (I use a plastic wash basin) where the collender is suspended. Cover with a damp towel and place in the coldest part of your fridge. If I am in and out of the fridge, I will put ice in the larger plastic container or a large ice chest.
I don't always use the best way. Keys: keep them out of fresh (sweet) water, keep a damp cloth or damp newspapers over them (think high humidity), keep them close to 35 degrees.
Caution using anything metal, it may rust.