K&F Concept 52mm ND2 to ND400 HD Slim Fader Variable Neutral Density ND Filter | eBay
Moment 52mm Variable Neutral Density 1.8 to 2.7 Filter 600-097
Rent a Tiffen 52mm Variable ND Filter at LensProToGo.com
Amazon.com : SmallRig 52mm Magnetic Variable ND Filter ND2-ND32 (1-5 Stops) No X Cross HD Optical Glass Waterproof Scratch Resistant Magnetic Adjustable Neutral Density Filter for Phone - 4215 : Electronics