A plastic sandwich bag makes a good pastry bag. Spoon frosting into the bag, then snip off one corner of the bag to create the size opening needed. Poke a tiny hole in one corner for an extra thin writing tip.
For the ideal disposable pastry bag, use heavy-duty, quart-size self-sealing plastic bags fitted with a cake decorating coupler and tip. The filling can’t work its way out of these bags because they’re sealed tight.