I wish I had more garage space for a table saw and a router, but ... hand tools will have to suffice.
Step 1: Supplies and Tools
I had some "left over" poplar planks ( 1/4 x 3 x 4 -- see picture for mm measurements) and several pieces of nice looking red wood of unknown provenance (5/16 by 1/4, 15 in long). Poplar is a bit soft, so I am not sure how it's going to withstand daily use. I expect to see scratches as time goes by.
Hard drive magnets still attached to flat brackets. I got a bit greedy and used 7 magnets. If I had to do it over (and I just might), I would have used 6 magnets to make the bar 15 inches long. I decided to leave the magnets attached to brackets, because the brackets provided the spacing between magnets. It was easy to glue them without magnets deciding to suddenly (and violently) to attach to each other. There is a way to separate magnets from brackets -- using a large set of pliers and brute force -- that would make the bar a bit less heavy.
Tools and Supplies:
Saw (ideally with a miter box)
Sandpaper -- 220 and 400 grit
Superglue or epoxy
Wood finish (or mineral oil)
Keyhole brackets for flush mounting to wall
Plastic bumpers (the kind used to protect cabinet doors) to help with flush mount and to protect wall
Step 2: Cut and Glue (and Cut and Glue)
Cut narrow pieces of wood (5/16 x 1/4) and glue (using wood glue) along the perimeter on the "face" plank.
Using "superglue" or epoxy, affix magnets as shown -- with the magnet side facing down. If your workbench has a metal top (mine does), it would actually help to keep magnets in place while the glue is drying. These powerful magnets will hold the wood securely against the metal bench top.
Using wood glue, affix the "back" plank. Use plenty of clamps but be careful if you're using poplar (it is soft)!
Step 3: Sand
Step 4: Finish
Step 5: Hang and Enjoy
But I didn't have it, so I made a reasonable substitutes from scrap piece of aluminum (not worth the effort, I bought the above for "next time") and glued them to the back. I also drilled holes (using hand drill -- no power tools, remember?), to allow for the hanging screws to use the keyholes.
Attach self-adhesive plastic (or cork) bumpers, hang this baby to the wall and enjoy!
Please take a look at other magnetic bar Instructibles -- this is by far not the first one. There are some very creative and beautiful ones out there.