I ended up fixing it to the steel below the bumper. I have a nice little drill that got in there which meant not having to take the bumper off. Otherwise grab a piece of flat steel bar and cut it down to fit in the bumper and suspend the light
I drilled through the plastic and into the steel below, then just mounted the lightbar. I had to buy a right angled drill attachment and that was it. Not had just a bit fiddly and time consuming to make sure it was as neat as possible. I wanted it to look like it was a factory option.