A dozen should be more than enough. String would also do. A few rocks. This is what is used in acetylene torches. More on this substance later.
Acetylene is a fairly dangerous substance.
For this reason, and for your safety, I recommend you keep all of the acetylene AWAY from any source of oxygen. This means don't let it get in touch with air. Make it VERY full. Now get put you garbage bag in the water and fill it with water. Now, being careful with the acetylene, take the bag out of the tub and tie the opening shut with the twisty or string.