The 2-perf frame was squeezed horizontally by 2X in post -- not expanded (except vertically I guess) -- to put it onto a 4-perf 35mm dupe negative for making scope prints. Anamorphic projection removed the 2X squeeze. So the Techniscope image was never expanded, it was the opposite, it was squeezed and enlarged (i.e. stretched vertically) to fill 4-perf 35mm and then corrected back to normal during print projection, i.e. the squeezed image was expanded back to normal. It was never a square image expanded to widescreen because then everyone would look very fat.
But why do you want a director's finder with a 2X horizontal image squeeze? Just to look at skinnier people and objects??? It serves no practical purpose because that's not what the final product will look like, no audience sees the image squeezed by 2X and Techniscope did not employ anamorphic photography so it wouldn't even give you a preview of what Techniscope looked like.
Besides, you could just get a lens finder (the type that is just a ground glass and PL-mount) and put a 2X anamorphic lens on it. But otherwise, if this is just for composition, a regular director's finder would give you the 2.35 frame lines marked with the anamorphic or Super-35 spherical focal lengths.
What would have some practical value would be a lens finder that took the 2X squeeze out of the image when an anamorphic lens was mounted to the finder, for director's who didn't want to look at a squeezed image -- many lens finders have that feature actually.
Unless you are talking about a finder that stretched everything out by 2X to make the image wider -- again, what would be the point of that except to correct for anamorphic lenses on the finder? To make everything and everyone look fat?