"portable, self-replicating 3D printer" you mean bacteria? I am sure someone is working on plastic eating bacteria but it might not be a great idea to have it loose in the world.
Many scientists/people only think about problem in the scope of the their discipline. Keep in mind this is the problem-solution knee jerk (treat the symptom) responses is how we got to our current situation in this world.
Oh we don't like horse dung, we going with cars now!
With bio-engineering such as this, many intergenerational horizontal studies need to be done before it even should be consider releasing in the wild.
You can engineer systems that effectively have an off switch (sterilized progeny with short life or a certain environmental lethality condition e.g. dependent on you constantly supplying certain substance) as well as even an API (e.g. built in features that enable integration with other genetic techniques such as insertion sites for cloning).