There are lots of ways to make self locking notches. I goofed with it a while ago and made these guys:
It represents 4 concepts, 1&2 can be backed out but 3 and 4 are fairly permanent. Once you click them together it’s almost impossible to undo 3&4.
I took some inspiration from the 50 digital joints project, found here.
The cross “x” joint in particular is relevant.
Edit: so the nice thing here is that the pieces are identical. 2 #1 pieces will lock together, 2 #2 units, etc. makes designing and assembling bulletproof.
In fact, later experiments proved that it was over engineered. You really need only one “latch” side on one half of the parts and it holds very nicely. It comes down to your strength requirements; the double latch double notch system as designed is remarkably strong, but the single sided “rounded latch” version was by far the easiest to assemble.