M5 Hex head bolt, modified by cutting off the head with 20mm of shank attached, and the end slotted - theoretically 1mm wide x 5mm deep. In truth that means cutting a slot with a hacksaw - nothing technical, and not necessarily even straight! In my case I used an M5 x 60 bolt and the other piece provided the other half. See next model. This model is just provided to show the shape/take dimensions lest my explaination isn't clear - do not expect a 3d printed version to work!