Using GND filters on wide angle shots
 

Being a 35mm wilderness landscape still photographer, I use graduated neutral density filters very frequently to balance light. I now have a Sony EX1 for video, which I love, but even my larger 4x6 Singh-Ray GND filters do not provide enough coverage for my wide angle lenses to get the shots I'm used to getting with photography. Are there much larger GND filters that I am missing? I haven't found a solution. Thank you.