Fit the kit, then spray plumbers expanding foam in there. This is what I did.
I reversed the car on a field. Back wheel fell down the whole so the rear bumper and side skirt where holding the car up taking the weight of the back passanger side. Not one crack!.
Driving down the M5 at around 90mph. Some of the screws some how fell out I guess. Back bumper on the drivers side fell off and was dragged for a bit. The fibre glass didn't shatter, just wore down.
So to all those people that took the piss out of me for using plumbers foam (which they did at the time) were 100% wrong!.