There is a model this is derived from. But I didn't found it again so there is no link to the origin. But every file was (re-)created by me with adaptions according to my needs.

printing

general

I used PLA, 0.2 mm layer hight, 0.4 nozzle

Everything is designed with only 0.1 mm gap. So you might need to wiggle a little bit to make it fit. 

hook

use 5 perimeters, 4 bottom, 4 top layers, 25% infill

arm

print 2 times. use 4 - 5 perimeter. For a nice look you can use 0 bottom and top layers and 25% cubic infill. But 4 layers and 25% infill is also fine. 

joint

use 5 perimeter to make it solid

assembly

Use the female joint part as a base. Stack one arm with the hole on the straight arm and the protrusion facing up on the joint. Than stack the hook - no preferred orientation and then the other arm with the protrusion facing down. Finally press the male-joint-part in.

(un)folding

to unfold, turn counter-clockwise and to fold turn clockwise