Sometimes DJI just makes it look like there is a clear filter installed, like on the original Air and I guess maybe the
Air 2S also. The
M2P does in fact have glass inside the OEM filter ring, and I assumed it would be the same as the
Air 2S and
M2P gimbal housings are virtually identical.
If that's the case, and the OEM filter ring is purely decorative with no glass inside, than anything from brands like Neewer, Freewill, PGYTech or Tiffen on the cheaper side, and PolarPro if you want the best, but I just took a quick look and I don't think PolarPro even offers a clear filter. DJI themselves doesn't even make a clear or UV filter from what I can see. Finding an actual clear filter appears to be very difficult as virtually all of them are labeled as UV filters, but don't worry about that too much - it's only in very rare scenarios that it could have a negative impact on your image and as long as the filter is decent quality, that's what will matter most. I highly suspect most of them come out of the same factory anyway, and a different brand name is simply printed on.
It looks like Neewer and PGYTech both offer UV filters at least. Again, make sure you are not buying anything with polarization also built in if you choose to also buy some ND filters.
Also, almost for sure, the front 'element' of the camera is still just protective and not actually part of the optical group making up the actual camera lens, but the fact that you can't remove/replace it still means it's worthwhile to protect.