How to Build a Cheap and Simple Variable Neutral Density Filter | PetaPixel
ND2 400 Neutral Density Fader Variable ND filter Adjustable for Panasonic Lumix DMC FZ72 FZ70 FZ50 FZ30 Digital Camera|Camera Filters| - AliExpress
Can cheap ND filters be good? The K&F Concept SN25T1 filter kit says yes
Amazon.com : JJC 2-in-1 58mm Variable ND Filter & CPL Polarizer Filter, 5 Hard Stops ND2-ND32 Adjustable Neutral Density Filter, Including Moistureproof Storage Case and Lens Cap : Electronics