Thanks for the idea! I have picked up several good tips from this topic :-)
Very nice idea! I'm going to mount a magnet to my bench this afternoon.
I use a Panavise 301 and a C-clamp base with a custom set of jaws (similar to the Stew Mac vise) that I fabricated from steel bar stock. The 301 has a ball joint that allows the vise to pivot and may also be set at an angle to the work surface, which is very useful IMHO. You may have to shorten the guide rods slightly as my jaws are slightly thinner than the originals.
The Panavise is not as sturdy as the Stewmac but has plenty of holding power for nut and saddle fabrication. Overall, it is more expensive than the Stewmac vise but I really like the ball joint and positioning versatility that it provides.
Jaw design is attached.