Ron at FtW has been doing an excellent series of posts about magnetizing the space marine vehicle chassis. He's done a rhino/predator example and a few people have mentioned razorbacks & whirlwinds, and a couple have challenged a vindicator.
Well, I'm positive Ron would do a better job than mine, but I wanted a quick & dirty vindicator conversion which meant not cutting away the front of the rhino hull. I decided to snap some quick pics to silence the naysayers. Here is the cannon itself. I just used plastic card to fill in the gaps. I did a terrible job trimming the sides of the plastic card though to fit the front of the hull.
Here is where I put the magnet - glued to the piece of plastic card so it will meet up with the magnet in the front of the rhino's hull.
Finally here are a couple of shots with it attached. As you can see it juts out quite a bit, but it doesn't really bother me - I'm just happy to have all the rhino stc options magnetized.
Here is my previous rhino magnetizing post.