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Remember that reflections off of the inside of a hand held filter can be a significant issue for hand held filters in many lighting conditions, and it is even more of a problem for solid neutral density filters that are in use to increase exposure times. So even though you do not like to mount the filter, a screw in ND filter can be a good investment for quality shots. The same goes for 4 by 4 solid ND filters, where a holder for them can help cut down on light leakage. The Lee Big Stopper filter even has some gasket material at the right places to help further cut light leakage when used on the Lee holder.
Graduated ND filters have less of an issue for light leakage when hand held, but still can have reflections. Besides light leakage, I happen to prefer a holder for Grad ND's so that I can position the filter the way I want, and carefully examine the wide open or stopped down view, and then adjust the filter position before shooting, or after a trial shot. If I hand hold, it seems like I have little control over what is going to happen.