@asian - instead of drilling and using screws to hold your diffuser to the bumper what I used to hold my aftermarket diffuser was bumper retainer clips. They are Hyundai/Kia bumper retainer clip that I found at advance auto. They just slide right over the diffuser tabs.
Here's the link to the retainer clips. I think there are 3 clips to a pack.
https://shop.advanceautoparts.com/p/...30d/11910672-P