The Magnetic Mount included in our package is a high quality and very strong 7 inch diameter magmount with a rubber boot to prevent scratches and a moulded stud where the aerial screws in and the cable comes out (better resistance to water than the cheap type with washers etc).
Unlike most mounts, the magmount does not need to make a metal-to-metal connection, as long as the piece of metal you want to put it on is at least a couple of feet wide and long, then it should work fine. This means that it can be fixed onto bonnets as well as roofs.
Obviously, magnets won't stick to alluminium but there is a solution for Defender and series Land Rovers - if you have access to the underside of your roof (i.e. without headlining) you can have another magnet underneath the roof, below the magnet on top of the roof, and these will "clamp" the roof (their magnetic attraction will pull them towards each other - with your roof in the middle) and hold on fine.
This is a method I have used a few times putting in temporary setups in mates' work 90's, for some CB-enabled offroading (much more fun than without CB's!).
The magmount is an obvious choice if you have a steel roofed vehicle (Discovery, Suzuki, Trooper etc) and want to have a temporary "fit it when you need it" CB system. Just plonk it on the roof, bring the cable in through a window or door jam (you may need to slit the door rubber slightly just to make a space for the cable to sit in, this relieves some of the pressure on the cable when the door is closed, and also helps to keep the cable in place so it doesn't "flop out" when the door is opened) plug the lead into the CB, and connect the CB radio to some power (maybe opt to have a cigar plug put onto the power lead of the CB set you're buying - it's only £2 extra) and away you go!