simple three-way rope lock, only locks once all three components are slid together. the top and bottom parts are held together using dovetail joints while the third part locks them and itself together. I probably should have made the slanty cube a dovetail as well to prevent it popping up.
print using the correct settings for your printer. I used 10% infill and 0.2mm layers. get a ~3cm m3 bolt and screw it first through the loose eyelet, then pass it through the part with the female dovetails, then add a spring and finally screw it into the small slanted cube. make sure that the cube is still within its guide when the spring is fully extended.
slide together the dovetails, then insert the locking pin from the bottom. you should hear a click as the pin mates.
pull on the eyelet connected to the locking pin and remove the pin. the dovetails are now free to slide out.