What you really want to determine is what path within the container is getting files written by Plex at that time, and map that to an external location. If it is not the /transcode path within the container then what is it? I also notice that the screenshot does not show the mapping for the plex config folder - where is that mapped to?
In terms is where to map a location for temporary files I would think that a pool is better than the array for performance reasons, and if you have lots of spare ram you might want to consider a RAM based location.