A Blender (2.8x, 2.9x, 3.x and 4.x) addon to create concave hulls (alpha-shapes)
Download a release .zip from Releases, then open it with Blender in Preferences, Install Addon...
You'll find a section called "AlphaMesh addon" in Object Properties. This will be active when you select an AlphaMesh object (you find them in Add->Mesh->AlphaMesh). Just select an object and then its particle system. The Alpha value defines how concave the mesh will be.