Cana anyone recommend a plastic wrench that doesn'e scratch the metal?

Once you know what size you need go on line to Snap-On and order the appropriate sized open ended wrench. Their wrenches are entirely polished and chromed. They will not scratch anything. Just be prepared to spend a bit more on them.  

Thanks for all the suggestion... I guess I'll just have to tape it over before tighten the post.

As long as you have the correct size socket, any type would do. You just don’t want it to slip, that will cause scratching. Put the correct socket on the post and tighten with your hand, no need for a ratchet. AudioQuest has made this.

https://www.amazon.com/AudioQuest-binding-post-wrench-Discontinued/dp/B0009XQUAM/ref=sr_1_1?hvadid=77996669303746&hvbmt=be&hvdev=c&hvqmt=e&keywords=audioquest+wrench&qid=1575396695&sr=8-1

@dill that was the one I was looking at but the inside looks like metal?

It won't matter, if you have the correct size, it won't slip so it won't scratch.
Any socket will do, you just need the correct size. I used a socket many times.