I'm using the Neewer, ND4/PL, ND8/PL, ND16/PL and they polarized as well and cheaper than Skyreat, around USD $30. I don't have any other to compare them against but 4k videos look stunning on my 4k TV.
Neewer 3 Pieces Pro Lens Filter Kit for DJI Mavic Air Drone Quadcopter Includes: ND4/PL...