That nut is huge. No open/box end big enough. My largest crescent was just a bit too small. Dug out my oldest tool, a 29/22mm flat Yamaha wrench from 1975 & still too small. Vice grips to the rescue. That gasket inside good for more than one crank down on it?
Water pump pliers, or Channelocks, have always been my choice...
https://en.wikipedia.org/wiki/Tongue-and-groove_pliers