Do you not have the option to earth the frame and the inverter to a suitable earthing point? Then you can create one yourself by driving in an earth rod.
This 2-metre earth rod is supplied including a clamp to secure the earth wire to the rod. Please note! The earth rod must be driven in deep enough that it stands in the groundwater in both summer and winter.
By coupling several earth rods together, an earth rod can be driven in deeper.