I have my cluster apart and the speedo "gasket" stuck partly on, partly off. I reproduced it in card stock (like the back of a notebook) using a compass to trace a pair of concentric circles, then scissors. didn't have a compass that would carry an xacto blade.
The "gasket" keeps the chromed ring from contacting the conical metal part with black wrinkle or spatter finish. If there's no relative movement between the two it might not be needed, but cars vibrate, and who knows how you can get another of the chromey rings that hold the face to the instrument so probably best to have them. Thickness is not really an issue, so long as they separate the cone and the chromey bit on the gage.