It actually fits pretty well, just a bit long at each end.
I used 6mm nuts and bolts (about 9p each from B&Q) I think I used about 8 along the length
Took me a couple of hours to do. Bumper off, rip old rubber strip, and line it up. I cut little access slots with a stanley blade so I could fit bolt though then did them up murder tight.
Didnt have to cut any slits, it fitted the contour of the bumper pretty well. I just pushed sides in and drilled holes through. My advice is to start with a central bolt right in middle, then go 1 left, 1 right, 1 left, 1 right etc. (not all along left side then all along right side if that makes sense)
The splitter aint going anywear, I have cought it a couple of times hard and apart from a sickening noise, no damage.