The front is supported by the trim panel with the visors. ( a bit tricky to put this back guiding the screws back in). Yes grab handles and the A and B pillars hows it up.
The rear it kept in by the two main side trims around the safari windows and the rear trim above the door.
Take care when removing the 6th and 7th seat grab handles as the have spacer blocks which could slip down inside the trim.
My board was a little worn where the rear air con and light switch sits. It was squishy and the fibre was a little broken. I used a hot glue gun and a small strip of thin plastic to repair it and make it stronger.
Also be careful of taking off the clips that hold the cargo nets. They can snap and did!. I used some washers and a screw to re attach them. Actually feels stronger..

