To make it so you can adjust the tension on the blade I took a 12” threaded screw and put it through oversize holes in the back end of the arms.
I put a nut on the bottom and a adjustable knob on the top.
The other end was a bit harder to figure out. I kept browsing over all of the fasteners at Home Depot, thinking there has to be a way to make a solid connection between the arms and the blades. Then it hit me and I think it is simple and strong.
I took a threaded rod coupler and drilled a hole in the side of the arms just big enough to put the coupler in snugly . I then used epoxy to keep it there. I was careful to keep one of the flat sides of the coupler parallel to top of the arm. It just will make the next step easier. When the glue is dry, I then drilled through the top of the arm right through the coupler. Next came a set screw and brass screw that I filed flat where they meet the blade.
Keep the knob on the top arm as small as possible, it gets in the way when using the saw.