The EGO 56V trimmer has an M8 nut that holds the head in place. I wanted to use a standard head sold for the Stihl line of trimmers since they are plentiful and tend to be more commercial and robust. The Stihl is a left hand thread M10 x1.

This print takes the Stihl left hand thread and fills it with the 8mm double D shaft of the EGO. You then secure the trimmer head with the 8mm nut from the EGO.