Please have a look here:
The idea was to draw with the Spline > Pen
Then use the Randomize Points (Mesh> Move> Set Point Values [Crumble Axial])
Scale certain areas of the point cloud to get close to the wanted shape.
Use the Sweep Tool to get quickly a visible result (Even Splines can be used in Volume directly… here I preferred Sweep)
I displaced the result a little bit
In Volume> SDF Dilate and SDF Erode as well as SDF Smooth
Note that in Volume Builder, the Voxel Size is crucial to keep it as large as possible but (secondarily) as small as needed.
Let me know if there is any question about this process. I’m happy to look into it.
(images: max 800w*600h)
All the best
Click thumbnail to see full-size image