mass-manufacture plastic
rjprins: I've studied industrial design for a few years, here is what I can tell you:Plastics is a very large subject and there are a lot of criteria that can be involved. You criteria apparantly are:  - Can be made in small series. (ie. no injection molding or extrusion)
  - Holds electrical circuitry.
  - Serves mainly the purpose of container. 
  (no exceptional stresses, no special features, 
  must be able to open and close in some way.)
  - Consumer product, so must have somewhat nice finish.
  - Should be tappable for screws?
  - Production accuracy within 1-2 millimeters?
  - Can be made in your own workshop?

Here are some small-series techniques you could look into:  - (Resin) casting
  - Dip molding/casting
  - Heat molding
  - Spin casting
  - Vacuum forming
  - Hand laminating or spraying

I don't know what materials are exactly applicable to what technique, but for your purpose some common materials are:
PP, PS, Styrene-butadiene, ABS (especially), PA6, Polyester (resin).Goodluck!