Few parts are required to homebrew this magmount.
A 5¼” floppy disk drive rotor, with its boss removed, serves as the magnetic base. A thin plastic sticker, covering the exposed face of the magnet, prevents damage to the vehicle paint surface.
The enclosure is a suitably drilled Melamine or Bakelite cup on which the SO-239 is mounted.
RG-58/U or smaller coax is used. A ¼ λ counterpoise of stranded insulated hook-up wire is soldered to the braid of the coax. This is a must in case the rig is to be kept isolated from the body of the vehicle.
The enclosure is potted with epoxy to waterproof it and make it base-heavy. The same epoxy holds the assembly in position on the magnetic base.
The driven element is a ¼ λ length of 1.6mm brazing rod soldered to the PL-259 pin. The space between the PL-259 body and the driven element is filled with epoxy to prevent water ingress.
This magmount proved its usefulness on many occasions when access to the vehicle battery was denied and a separate battery had to be used.