Lots of good inputs here.
I ended up using a plastic license plate holder that I got from Aerostich and used four mounting screws:

Also, the nut and bolt I was using to plug my stop-drilled hole (as shown in the OP) ended up interfering with the bike's license plate bracket, due to their location. So, I replaced said nut and bolt with an aluminum pop rivet (with a 3/16" diameter mandrel), as shown in the above photo. The head of the pop rivet had a low enough profile so as not to protrude past its recess hole I drilled in the plastic license plate holder, just behind said rivet.
The reason the pop rivet looks the way it does is that, after installation of same, since it was still a wee bit loose, I laid said rivet against an anvil and gave it a few taps with a big hammer.
You can see the crack running under the red reflector at the upper-right corner of the license plate. Where the crack hits the edge of said license plate, I applied a narrow length of stainless steel tape, bent it over along the long axis, and then hammered it flat.