Grill
Well, you can use wire ties... I see a lot of guys who slap gutter guard on there with wire ties, but it looks bumb to have the end of a zip tie all sticking out.
I also have gutter guard stuff, but I got a body shop to remove my stock plastic lower grill and install the new one. Gutter guard is fine on that, it looks great.
I would say, if you dont want to cut up your car... make a card bord template of the space, cut grill to match, and leave 8 little tabs that you can fold back. zip tie that tab to the existing plastic grill... that way I wont have to see your zip tie sticking out of your grill.
Take the whole bumper off while you do it... that way you can get in behind there.