Tetrapod is the popular shape used to make breakwaves. Already seen on thingiverse, I just needed to redraw the exact geometry so the 3D model is also available here.
From that I made a mould that one can stuff with concrete. Make it quite liquid & push, stirr, with a metal wire.
Print three identical parts and assemble the mould with 3 M5 bolts. might work with clamps.
If not resized, the tetrapod will be around 30mm high.
Please post your makes