High Impact Polystyrene is a very common plastic material used in hundreds of enclosure applications here at Envision Plastics. Stryron 484 HIPS is UL listed with a UL94-HB flammabiltiy rating. It is a very durable and economical material stocked at Envision in various thicknesses and available in any color as long as it's black. Custom colors are an option but minimums apply for custom material extrusions through distribution.
This material is very common for low volatage electronic enclosures which do not require a higher UL94-V0 rating.