The idea is to stress the metal up into a bowl shape without having to use any welding or complicated tools. By making a few slits here and holes there, you can bend, fold, and pin the metal together pretty quickly with some tin snips and a drill.
Assuming the sign is free, only a couple bucks' worth of fasteners are needed. Tin snips or a Dremel, pliers, a drill, a wrench, a screwdriver, and a ratchet would be handy to have close. Only takes an hour or two; most of the time is actually laying out the cuts more so than the actual construction.
It uses the same basic principles of this chair on a smaller scale: http://www.instructables.com/id/Nine_Square_Chair/
For sale here: http://www.etsy.com/view_listing.php?listing_id=28562050
Step 1: Drills/Cuts
Decide how big you want the bowl to be -- the walls on this one are about 3" wide. Measure 3" in from each side and run lines to create a smaller rectangle offset inside the original perimeter.
Then measure 2" down from each corner in each direction. Connect these lines so each corner is "clipped." Measure the midpoint of the cut line. Connect that midpoint to the corner of your small rectangle. This is the center cut line to create the triangular fins at each corner that turn the sign from flat to bowl-shaped.
Clip off corners. Drill a hole at each corner of the inside rectangle. The corner holes on this bowl were 5/8"; choose whatever bolts you have handy for the bowl's "feet" and drill accordingly.
Then, bisect each triangular "tab" you have now created, measure an inch or so down, and drill hole 3/16" or so on each side. Refer to photographs for placement.
Cut down the centerline of your "tabs" into each corner hole.