I'll just inject the following, being that I'm an alleged RF jock and EE:

The whole "ground plane" issue doesn't kick in until you have a ground that extends at least a one-quarter wavelength radius away from the centerpoint of the vertical antenna. When you consider that the CB band's wavelength is about 11 meters, a full wavelength is about 36+ feet, and that makes a quarter wavelength 9 feet.

So... unless you're mounting the antenna in the middle of a patch of sheet metal or metal screen/wire cloth that extends outwards in all directions nine feet... you're really not dealing with a ground plane. You might be dealing with a ground reference, assuming you've grounded that circle of aluminum well, but not a ground plane at CB frequencies.

I'd just make sure that the antenna is mounted on something metal that has a firm grasp of the ground potential of the vehicle. I've seen more mobile antenna installations that had problems due to poor ground bonding back to the radio than the lack of a ground plane. With the amount of plastic & fiberglass in equipment and vehicle body work these days, mounting an antenna on the fiberglass will mean that you should definately run a ground bond from the base of the antenna to the vehicle or equipment chassis. Do not assume that the braid on the co-ax feedline is going to do it for you -- it won't. I won't delve into the RF fields and waves math here as to why not, just trust me -- putting a vertical antenna on the end of the coax feedline without a ground potential at the base of the vertical will give you strange results.

Copper (or nickle-plated copper) braid is better than stranded wire, and stranded wire is better than solid wire for this type of grounding job, and using braid becomes more important the higher you go in frequency.

For a VHF antenna, now if you mount the vertical in the middle of the roof or similar big flat space of sheet metal, you have something approaching a ground plane. You definately have a ground plane at UHF frequencies.

But not at CB frequencies.

There are antennas that don't require a ground plane to function, BTW, but they're not as simple, they cost a bit more and ultimately, you won't notice the difference at a quarter mile. 10+ miles, yes, but not a quarter mile.