Headrests on rear seats,sit on the rear shelf and fit over the seat top, no mod is req.
It is poss to mod a S1 front seat but if you mess up cutting the slot in the top of the seat for the hole,its knackered it!
Its far better,and a lot less hassle to find some good early S2 seats in the same colour and style " flat pleat " with the headrest slot already in them!
Seats are out there quite cheap if you search!
I got a complete low miles seat set in bronze flat pleat with door cards and other trim for £80.
And recently I got another set of really nice bronze ones that I intend to fully restore for £40!!