Again, as metioned above, the results may not be optimal.
But, given a little consideration, I think it is do-able as follows:
- You can use a square iron or steel base-plate that the antenna magnet will stick to.
- Drill a hole in each of 4 corners of the plate for attaching radial copper wires. The copper wires needs a firm clean connection to the base-plate.
- Each copper wire radial arm needs to be about 18 feet long. 18 ft is ~ 1/4 wavelength at 27 Mhz CB band. With the 4 radials extended full length from each of the 4 corners of the base plate, this will create a ground plane for the antenna system.
The weakest link in this scenario may be the vertical antenna component itself. As mentioned, antenna elements generally need to be around 1/4 wavelenght for best effeciency. The vertical elements of Mobile Antennnas are generally far less than 1/4 wave (~18 ft.) and thus will never be as effecient as a full 1/4 wave vertical element.
Go ahead and try it. Experimenting with antennas is half the fun of radio communications. It may not be optimal, but it may be a cheap "functional" option, if implemented with care. You can decide afterwards if investing in a commercially manufactured, full size, base antenna is a better solution.