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