Fill mesh with color / draw texture on box mask

Hi,

With the help of box mask I can now make parts of a mesh be transparent. It works amazing, but has a slight problem, players can see inside the mesh.

So I was thinking whether it is possible to fill the mesh with something or draw some texture on the box mask faces to prevent players from looking inside the mesh.

Here is a screenshot of what I mean:
image
Instead of red lines there should be a texture or color or whatever.

Does anyone know how I can achieve this?