My daughter added this to her Black Widow costume for Halloween. There is a place for straps to attach on the back side of the model. I think she just used a length of material and then pinned it in place. It was printed in red ABS, then she painted the outer part gray, and the center black.

There are two versions posted.  On the first version (marked "orig"), I didn't do my math right, and so the curves on the sides are not concentric.  The second version has concentric circles.  However, since this is decorative only, pick the one that you think looks best.

Disclaimer I do not own the rights to the Marvel Universe. However, I made this model from scratch, by looking at pictures online.