split ND filter type/reccomendations for Yellowstone

Just an advanced amatuer here. On my way to Yellowstone in July. Im looking to get a split ND filter for my trip. It will be used primarily with my new 24-105 L 4 canon lens. Ive seen the 4x6 plates at various strengths and the screw on round types. Id rather not use a holder on the lens. Can i just HOLD the plate in front of lens if i get that type? or should i get the screw on filter type?? Looking for easiest but effective

Which type do most of you reccomend, and which brands and Strength are probably best for my trip.

Any advice is appreciated

acadia 5/09
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Scott