CNC machining an enclosure will be extremely expensive, you will basically be machining away most of the material from a solid billet of plastic. In addition to this, snap-fit components are usually quite small with undercuts that will be almost impossible to machine, especially with plastic. The thin walls you mentioned will also be difficult to achieve via machining. So I would definitely advise against CNC machining this part.

3D printing is definitely the way to go, you will have to strategically place the snap joints so that the layer lines are not aligned with the direction of the applied force, or else they will break. You also need to choose a material that can work outdoors like PETG or ABS for example. The production volumes are also very achievable with modern 3D printers.