With the above said, the best option to my mind would be to 3D print and finish high-quality moulds (in titanium or steel) for use in stamping out aluminium case components from sheet metal.  I see modular design to be important primarily for the internal component selection and layout inside the laptop body rather than for the body itself, which—to my mind—should be designed for longevity, strength, rigidity, and good looks (important for attracting more people to buy the product over time).    

Lost-wax metal casting has also been effectively demonstrated with 3D-printed thermoplastic plastics but to me that seems like it might be a less environmentally friendly option (as compared to the above) despite possible reuse of the lost wax (the plastic). However, it might be cheaper and a more-viable option for such small manufacturing runs.

With regard to the choice of material, I would suggest the following order of preference: 1) aluminium (recycled if possible), 2) titanium, 3) sustainably grown biological material, 4) plastic.  Aluminium and titanium are highly recyclable (aluminium almost infinitely so) while (non-biodegradable) recycled plastic almost always requires additional plastic feedstocks—resulting in perpetual growth of plastic waste.  

It can be tricky getting truely environmentally friendly biological materials (such as bamboo) from non-western countries.  The reason is that the demand for the materials often leads to deforestation of native forests in order to grow the bamboo, palm trees, etc.  Sourcing materials (such as sustainably grown hemp, flax, or timber) from the few countries that have strong regulatory schemes would be ideal but typically comes with commensurately higher costs.  Also, biological materials are not likely to be well suited to the material specifications needed for a laptop chassis.  Note, too, that engineered products (like bamboo floorboards) often incorporate a large percentage of some pretty nasty glues.

My proposed ordered list above discounts the carbon-intensity of materials manufacturing, as well as the damage caused from the mining process, but working out such things is extremely complex and requires detailed audits of source material, material suppliers, and processing manufacturers.  It also changes from country to country and from region to region within certain countries.  For this project, I think it would be necessary to look at long-term material use and reuse.  Aluminium, although requiring lots of (possibly dirty) energy to produce, is also an exceptional store of energy due to its capacity for perpetual recycling.  It should also meet the fit, finish, and strength attributes required for the laptop housing.

https://voodoomfg.com/  (Volume 3D-printing an option.)

https://www.quora.com/Who-are-Shapeways-competitors (A discussion of Shapeways and some of its competitors.)

Cheers,

Bluey

P.S. Without getting into the complicated world of finite element analysis and minimising points of high strain in the design, I would recommend well-rounded internal edges wherever possible if/when designing a chassis case.  This will reduce high stress concentration in those areas.  The sharper an edge or corner (e.g., on a rectangular hole) the higher the stress concentration in the material will be at that location.