if it’s the last one, you can add an aluminum fitting to the end of the siding, or a fitting of some other material that handles outdoor conditions. By doing this you hide the end of the wall from the elements, it is durable, easy to replace and if you choose the right one it can look good as well. This will also enable you to just remove the rotten part in the bottom of the siding, unless the rot has spread deep or high in the construction, as the fitting will close and blend the cutter part. Just make sure to leave a small opening at the bottom of the fitting, to let the water escape, or you can seal it properly, as the fitting will not take water damage like wood, and is more easily to replace if needed.