I didn't like having the holes for the front license plate in my bumper. I was looking for black plugs that I could use to fill the holes, but was only finding plastic ones. While those would have worked I really wanted something that better matched the bumper. I found these.
The plugs are metal and come painted gloss black. They blend in quite well. I've used plugs like this in the past on my motorcycle. I ended up ordering them from a local bike shop. They're from Gardner Westcott. Part # 43571-GB. 3/8"
They also come in flat black and chrome.