Sorry for my english...
I make two version of my harm. One with 5.6mm nut hole and one with 6mm nut hole.
I include a test cube to help you with your choice between 5.6mm or 6mm. Print it without supports and the number on top. For me the 5.6 is perfect.
To assemble this harm you need:
4x M3 16mm bolt with lock nut
2x Wood Screws 6X3/4