I built a Hellhound in the small window of free time I was allotted last night. Of special interest to me was determining how to magnetize it so I can switch out the hull-mounted gun and the turret, making it interchangeably a Hellhound, Devildog or a Bane Wolf.
The hull mounted guns each received a magnet drilled into its back. On the actual tank, I superglued a flat-headed screw on the inside. Please do this before fully assembling the hull and spare yourself the awkward task of holding it in place with pliers from the outside (and having to fish it out if you drop it) until the glue dries ... like I had to do. 
The actual turret barrel was pretty easy. I carved out a hole in each side of the turret so a small-headed nail would fit within the barrel. The magnet is glued to the end of the peg on the barrel tip. Here are the results:
