The hole for the stick has a diameter of 24 mm. When printing the coupling piece you need to insert a M6 nut. To do this, you must carefully remove the support in the hole for the nut, making sure that the piece does not come loose from the plate. 
To assemble the nut cap, I screwed a nut onto an M6 bolt and placed it in the nut cap. Then melted it closed with a soldering iron and a piece of filament.