Snap lock mechanism that can be scaled and changed in a wide range of sizes. Works very reliably, and gives very positive locking. It works best when the arms has a reasonable length compared to their width.
The version shown has a 32 mm c/c distance between the mounting screws in each element, and a 35 mm c/c distance between the screws in the two different pieces.

Should build very easily. For best strength, make the parts in the orientation shown with regards to the Z-axis.