I used to design these a long time ago. Aluminum was always used, onboard device drivers necessitate good heat rejection and plastics won't get you there.
Wall thickness was controlled by manufacturability 95% of the time, not by mechanical loads. We were pretty much always going as thin as could be allowed by whatever casting technique we were using for that particular part.
There were a few cases with extreme conditions for things like mining equipment where mechanical loads (mainly vibration) were severe enough to require additional material beyond the manufacturability minimums, but these cases were rare, and often wound up using additional techniques to get PCBs to survive (100% epoxy fill, capacitors welded to cases, etc).