Why not use chrome hole plugs or covers. You can find them at most of your local hardware or home improvement stores.
Shop Hole Plugs at Lowes.com
When i changed the rear signals to the Kury Mini Chrome Bullets there
were two holes left by the old signal and a slight recess in the cover (for the old signal) much larger than required for the new signals.
I went to Ace hardware are found two chrome washers and two stainless steel washers (for the inside).
I brought one of the side rail covers in with me and found washers that "filled" the entire round "recess area (you'll see what i mean when you look at your side rail) while leaving a hole to fit the new signals, use the stainless washer on the inside surface of the rail cover.
in your case you could do the same thing except instead of installing a signal, install a license plate bolt (or slightly shorter bolt) and nut and use a chrome cover over the bolt head (common, i have a bunch i a drawer).