Just an M8 bolt to fit with steair's IKEA Markus armrest mount cap, available here: https://www.printables.com/model/130117-ikea-markus-armrest-mount-cap-fabric-clamp/comments/1530815

• Head: 5mm hex
• Height: 15mm 
• 1.25 pitche

Of course, it's weaker than a real metal bolt, though I find it sufficient in the case of the model above. Still, do not apply too much force when screwing it in.

Printed in PLA+, 0.12 mm height, and 100% infill for strength.