Would a polymer loaded with graphite powder be good enough for your application?
For instance some boxes for electronic equipment use them. They come in black or in black, are reasonably sturdy, are perfect against electrostatic discharges and provide some shielding against electromagnetic interferences.
For reasonable stability under oxygen, humidity, light
plus decent mechanical performance
plus reasonable size and price
it's nearly the only option. You may replace the graphite by metal powder or whiskers.
An other common practice is to metallize the surface of plastic parts.
Most conducting polymers are lab curiosities that exist as thin films and can survive between two glass plates for the duration of an experiment.