SetFactory("OpenCASCADE");
Block(1) = {0, 0, 0, 10, 10, 10};
Sphere(2) = {2.5, 2.5, 2.5, 0.05};
BooleanDifference(3) = { Volume{1}; Delete; }{ Volume{2}; Delete; };

inner() = Surface In BoundingBox{2.449,2.449,2.449, 2.551, 2.551, 2.551};

Physical Volume("my volume") = {3};
Physical Surface("my inner surface") = inner();

Mesh.MeshSizeFromCurvature = 30;
Mesh.MeshSizeMax = 0.2;