These were a straight lift from the P5 MkIII and P5B. They are an oblong aluminium casting, open at the bottom, carrying a festoon lamp. There was a particularely nasty push button on off switch mounted next to them which invariably loses its push button with age!, so I wouldn't replicate that bit! They come up on e bay periodically, often as part of a P5 head rest. There was also a rather heath-robinsonish assembledge of suzzy wires, plugs and sockets to connect the headrests to the seat and the seat to a socket on the transmission tunnel. I would'nt replicate those either if your starting from scratch!
Chris