Photos and tutorial from Harold Brasier, thank you for sharing!

Material:
1-cutter
2-pvc pipe (but ultimately possible to make holes without them)
3- a wooden support (best in filmed CP to avoid covering it with mortar)
4- three bombs of expansive foam (two would have been enough for this type of decoration)
5- 15kg waterproof mortar (several layers)
6- epoxy resin 123 special aquarium