i did this to my rs about 12 years ago.
think i got some rear headrests from a orion iirc.
then fitted some brackets to the back of the seat and slid the headrest legs behind.
was a bit of a bodge from the back but you could do a tidy job.
think i used the little bracket like you get on a back gate sliding bolt.
the bit the bolt slides into lol.
did the trick