Interesting. I just went through this on a US-Spec Z4 (e85) and had to purchase a new license plate holder. There was a template for drilling into the stock bumper. So I made two small pilot holes in it using the template.
The black plastic license plate holder I used small nylon (rust-free) screws with a washer and a nut on the back to affix. (Had to pull out one of the grills to get my hand in there easier. Purchased from my local hardware store.
The license plate itself I used short bolts (make sure they're not too long or it'll hit the bumper itself and ruin the paint under the plastic bracket. I put a cap on top of the bolt to finish the look. I couldn't decide on a color so I tried two different ones until I make up my mind. The front has black covers the back of the car has white.