Ball joint that is boltable between things that fit for you. Bolts and nuts M6.
The smaller holes are for little pins that prevent the addons from turning. I used M6 set screws. These holes give enough "hold" for the screws. Just screw them in there. :)

Remark: Inventor likes to scale the stl parts. So make sure to check the scale when processing it with your slicer. (I usual need to make it 10x bigger.)