Hey Falcon1,
You can use 2-2.5 gallon zip lock bag (should be carried in most grocery stores) a lot of the time what I will do after I rub my Boston Butts down is I'll wrap them tight in saran wrap before putting them in the fridge. This will partially cure the meat, push those spices in a little more and make the meat just a little bit more tender. But ziplock bags will also work just fine. Just be sure you pull it from the fridge long enough before smoking to allow the meat to get up to room temp. Good Luck!