OK, step one is to find the radius. Your caul is six inches long and it should stand up 1/8" at the center. In geometry language, you have an arc that is six inches long and it's sagitta is 1/8". First, a bit of a simplification: this is a very shallow curve so the length of the arc and of the sagitta are just about equal or close enough for this situation.
From Wikipedia
Sagitta (geometry) - Wikipedia
the formula for the radius, given the arc length and the sagitta, is given by:
So your radius is 36 1/16". Your ten foot radius was a bit over so it is easier to do in what is probably an available space.
As for how to make it, I would be tempted to use wood working tools and techniques. Clear off a work bench or put a sheet of plywood on some saw horses and use a sacrificial wood piece under your aluminum. I leave how it is fastened down to you, but a couple of screws in countersunk holes would be my choice.
A 36 1/16" radius arm can be used to swing the router in the needed arc. Here is one way of doing this:
How to Rout a Large Arc | Popular Woodworking Magazine
There are numerous others, just Google "cut large radius with a router". Use a new carbide bit in the router and some WD-40 for cutting fluid.
WEAR GOGGLES! I would also wear a shop apron as the chips are going to fly.
Note: I can't believe that the word "sagitta" was not in my computer spell check dictionary. But then neither was "caul".
This is kind of an oddball project that has been handed to me. Material is 1/4" AL. One inch wide by six inches long. It is to be used as a caul for bridge work. If the caul was sitting on a surface plate the center would be 1/8" up from the plate. I have no idea what the actual radius is and it would not do me any good to know anyway because I can't think of any way to machine this. I would need a fly cutter with a ten foot radius or something like that. I thought of making sure all sides are parallel and then putting it upright in a press. At some point it is going to bend one way or the other. I could set up an indicator and see how much over-bend it takes for it to spring back to what is needed. I then thought to make it from steel and heat it on one side causing it to bow slightly. I know this sounds simple but it has to be quite accurate. Any ideas? Thanks.