What everyone has said about tension wrenches is correct. I have found that the variety of sizes of wiper stiffener is really interesting, there are at least 7 or 8 different widths, all of them just in the right ballpark for making tensors, and they also have the rounded smooth edges that will prevent them from digging into cylinder walls at the bottom of the keyway like some rough cut sharp edged metal will
I recommend the shorter versions of the tensor, about 3 inchs long, with a twist just at the last inch, to make a nice flat place to put the finger on. Wrenchs that have a twist close to the 90 degree bend tend to be flexy, and this can give less feedback to the finger.
grab all the used wipers you can find in the scrap barrel at the your mechanics. Strip out the stainless stiffeners. Stock up because they're making them out of plastic in the future.