Visually close InstancedMesh clipped by clipping plane

I want to have spheres inside a box made of clipping planes. The camera looks into the box from the outside.
The box has periodic boundary conditions (a 3 torus geometry), thus spheres get clipped a lot.

To model the spheres, I use IcosahedronGeometry with MeshStandardMaterial, to create an InstancedMesh.

It works fine, but the fact that the spheres look hollow when clipped bothers me a bit.

Can someone help me visually close them?
Any help would be appreciated.