Hello Polzan!
There is a way to do a custom shape on a volumetric volume!
It requires a (free) Plugin found in the following link
https://forums.unrealengine.com/t/realtime-simulation-and-volume-modelling-experiments-livestream-plugin-content/91202
This pluging is explained in detail in this video
You can also follow this video instructions in order to achieve a volumetric shape from a texture
Hope this helps!