I cut and bent the sign in such a way as to try and capitalize on the inherent properties of the material, namely its tensile strength. Since I designed it as I went along, the substructure is a little weird. I just kept adding pieces until it stiffened up. With a little more forethought, you could organize the structure better, but I kind of like how it developed organically, and the final form reflects that.
That said, it is still surprisingly flexible, and conforms to your body some when you sit in it. At about 13" off the ground, it is a low, lounge-type chair. Given that the wood is already weathered, and the sign was meant to be outside, this chair is great for a porch, patio, balcony, or deck. Originally 2' x4', the dimensions aren't ideal for a chair, in that the seat pan is a little long tailbone-inside-of-knee wise. However, when you sit in it, the big 45-degree cutbacks on the front edge are where your legs go, and it works out fine that way. It is really comfortable and incredibly lightweight -- you can pick it up with one hand.
I don't expect you to find the same sign, or the same dimensioned sign, so I'm not going to include any dimensions. In true readymade style, I measured everything just by holding it up and eyeballing. You can do the same . . . .
Step 1: Bending the Metal
I marked the sign into two 2' square halves. Off that center line, I made two sets of two 45-45-90 triangles with legs of 5", 5", and 7". Snip down that center line 5". Then carefully line up the hypotenuse with the edge of the workbench and hammer it down with the mallet. To get the cleanest bend, hit right on the edge, not down at the bottom of the piece. Repeat for all four triangles.
I did the same at the corners, but those triangles have 8" legs. For what will eventually be the back, hammer them down all the way flat. Leave the triangles at the front of the seat at 90 degrees or so.
Last, bend the whole thing in half along the center line by pulling the centerline triangles together until they completely overlap. Drill a hole through both and bolt through to hold the sign that way (see the last photo of the intro to see those bolts up close).