For the cake-only one boxed cake mix was used to bake the cake, two will be too much, you wont be able to close the lid.
This can be made for smaller servings and work as a party favor! Amazon has several foil pans with lids in bulk.
You can use 12inch piping bags but I would recommend 16-inch piping bags because you will have to refill the 12 inch at least once.
This technique uses more frosting than you think it does so you will need to have several cups of buttercream.
I wouldn't recommend using store-bought frosting as it will be too soft and not hold its shape, you could, however, use store-bought buttercream (which can be found at craft stores, restaurant supply stores, and Walmart
To clean your piping tips, boil them in hot water to get rid of any butter residue. I also run them through the dishwasher after boiling.