A filter holder is not absolutely necessary (the money you can save), you can filter that is also keeping up with the hand and thus react more quickly variable. But be careful when handling, do not move directly to the lens, otherwise could occur fine scratches.
For smaller objective diameter filter size is 85 mm still sufficient for larger diameters can filter the border appear in the image. One just needs a little bit careful. That is why I would prefer the broader filter in next time.