pretty sure urea formaldehyde or ABS are the weapons of choice....you want electrical resistance, rigidity for support, tough (for protection)  UF is a  thermosets (as mentioned above)...compression moulded i'd say.. (2 piece old skool plugs)  ABS is a thermoplastic...injection moulded (those..pre-fitted plugs)  Fuse holder is usually copper....good conductor, easy to process   i do materials science   adam