Just yesterday I re-installed my Euro/Brit/rest of world fitment Zunsport lower IC grille (the M-Sport logo stenciled one), without using the factory plastic slat insert which it is supposed to self-tapping screw into.
This can ONLY be done with zip-ties (I used black, weather proof ones, and you cannot tell they're there unless you get right up close to the grille) wrapped around the vertical bumper 'posts', at least from the front, with the bumper cover still in place.
It still does not fit perfectly (like I am taking for granted these USDM modeled ones WILL, regardless of how they are mounted), but at least it is centered now, and not pulling away at the corners, like it was when it was attached to the factory insert.
There is also a tiny bit more air hitting the IC now, albeit only on the plastic end tanks (still on the factory IC), without using the factory grille insert.