52mm Variable ND Filter Kit for iPhone 13/13Pro Max – Photo & Video Gears
Amazon.com : Cellphone Camera Lens Filter, ND2-400 Neutral Density Filter, 37mm Cellphone Photography Camera Lens Filter with Adjustable Clip, for Different Brands of Mobile Phones : Electronics
NiSi True Color ND-VARIO Pro Nano 1-5stops Variable ND Filter for IP-A Holder | NiSi Optics USA
Mobile Video & Filmmaking: ND Filters for Your Phone? - YouTube