Pretty easy to fit to the Boxster actually .. the panel that covers them is a single phillips screw half way down , pull the panel outwards at the top towards the center of the car , that releases a fixing then pull the panel upwards , theres 2 fixings that have to slide upwards or they can snap.
Very early 986 has an 8mm nut at the very top that needs to be removed , its easy to see if fitted though.
Once removed the seat belt is accessable to replace.
Red belts with black interior works well IMHO.