Hi Fat,
I have done a similar thing for my (still to be completed) ute. These bits were carefully fabricated by the panel beater to sit behind the pillar....its a bit tricky getting them in, dangling them from a bit of wire but then are then held in by small screws top and bottom.
I bought inertia reels from Rare Spares which I am not entirely happy with but....we made up mounts for these to prop on the floor which just clear the bench seat and again are contoured a bit to sit flat.
I ran all of this by the engineer as we were doing it and he seemed pretty happy.
Jonno