Adjustable ND's are very heavy filters as they have double-polarizers (glass). Plus, they can give you a dark "X" pattern if dialed too much. Better off getting the single ND16 or ND32 depending on your ambient light. The single ND screw-ons are probably the lightest ones out there that won't overload the gimbal as they have no separate moving parts either.
Personally, I find the polarizer doesn't work too well with the extreme wide lens on the Phantom as it darkens a portion of the sky and the remainder is too light on both sides. If you fly the same direction it's not as bad, but swing it around a bit and the sky gets a bit messy in exposure differences.
Maybe
DJI Phantom 4 Filters - Cinema Series - Shutter Collection