I installed a tusk tail light on my wrr today.
I took the rubber mount off the stock signal lights, this leaves a small bolt sticking out of em, drilled a small hole in the side of the tusk light and reused them.
I mounted it quite far back to try to keep me cleaner, you could cut two inches off the front of the lights mount and have it right under the fender if you like tho.
I drilled holes down through the rack/stock plate holder mount into the tusk light, the racks bolts hold the tail light on and I used the two smaller stock light mounts behind the rack mount, I have 6 bolts holding the tusk light on this way.
I've also had a DRC edge light on my bike, this one is just as easy to mount if you can drill a few holes.
I also sealed all the way around the light and where the wires go into it with black rtv silicone.
For offroading I prefer this light to the edge light as the bottom of the light is covered with plastic and it will keep more mud off the light so cars can see me.