Another option for either a Circular Polarizer or Neutral Density or even a graded density filter with very wide angle lens is to use a Cokin type filter. Get the largest (X-series I believe at the moment) square (almost square) filter in the type of material you want and either manually hold it in front of the lens, use the gaffer's tape trick, or get a 'standoff' holder, not the system holder necessarily but a rod/shaft mounted frame that attaches separately. These are used if the front element of the lens extends beyond the lens tube as is the case with some super WA models.

There are different grades of CP and ND that are made to lessen the banding effects from using a large filter piece. Less expensive (still a lot money sometimes) filters not made correctly will display that gradient effect when light hits them at certain angles.