Yup, ya gotta watch what ya brine in, some plastic bags er treated with pesticides an such. If it ain't a food grade plastic (bag er bucket) please don't use it. Maybe nothin would happen, but I don't wanna find out either.
Stainless steel is fine, don't use aluminum, an as long as ya can keep the temps from 36° - 38° it will brine just fine. Get much below 36° an it slows the brine process, get lots lower then that en it will almost stop. Don't get much higher then 38° cause a the risk of spoilage.
I can get some real large zip bags from time ta time from my food supplier that work well. I use a frostin bucket I got free from the local mega mart bakery. Rinse out well an work great, has a tight fittin lid on it to.
Also, some folks brine in a well washed cooler.