I binned off the black plugs because I thought they'd fall out and wouldn't last long, and used some big self tapping screws with a washer behind them. Drilled a small pilot hole to start them.
P.S. You did it the right way, fitting the rears first, so you can get the fronts the same height. I did the fronts first without realising the rears were a set height, as a result my fronts are a fair bit lower!

Might take them off and re do them one day, need a new front arch lining anyway so might as well get 2 and do it properly!

Another tip is use some stainless bolts/screws, as mine have gone rusty now!
