Schneider Optics (B+W's parent company) makes a 100x100mm ND1.8 (6 stops), in their MPTV line (motion picture/television) - B&H link. Note that they're Schott glass filters (they're the only ones that I know of who make glass square/rectangular ND/gradND, vs. resin. The issue is see is that they're twice as thick as the Lee resin filters. I know the Lee holder can take them with an appropriate adapter, but I don't know if you can use a 2mm Lee and a 4mm Schneider in the same holder at the same time.
Just found a HiTech 4x4" resin ND1.8 that will fit a Lee holder - B&H Link.
Hope that helps...
Thank you for the help Neuro, Hitech have a 6 stop one, I can stack that with the Big Stopper since it comes with a 1.5 mm gasket fitting the Lee system. Lee themselves are also considering releasing a 6 stop filter in the next 3 months or so they tell me. Hope vignetting will not be an issue, but I use an 82 mm ultrawide adapter, so hopefully not.