I've asked this questions before, but still haven't gotten a good solution yet. Attached is my SolidWorks assembly. It is a simply assembly of spheres placed in a casing. I need to fill the voided space between the spheres in the casing so that I can change the material of it (to make it air, water, mineral oil, etc.). I need to be able to do this so I can import the model into a optical modeling software to do ray tracing of light. Any ideas on how I can fill the voided space in an assembly?