Tested a theory to-nite to see if hard drive magnets will work through non ferrous material, as you can see there's 2 redundant hard drive magnets jury-rigged to a bit of steel bar, with a piece of 3/16" gunmetal and a piece of 1/8" steel plate, it also works with 1/4" ally as well ( not all at the same time though ).
(http://farm5.static.flickr.com/4114/4901733965_c5216bd67b.jpg)
So the hunt is now on for a suitable piece of ally bar to sacrifice to the god of swarf production, epoxy that of magnets into a recess do it don't fall out and then were in business.
TL :)
Now to settle down and watch 'The Patriot'