Don't drill the seat, that hole is backed by a nut welded to a plate and bonded inside the structure of the seat.
I would call brey Krause and buy one of their GT3 seat to OEM slider mount which should be exactly what you want. Solved
and the right and safe way.
R-9079 Side Mount Brackets for GT3 Race Seat (fits manual stock sliders)- 911(1999 - present), Boxster, Cayman
Then you could save the original side mounts or optionally sell them and possibly recoup your cost.