Can you send me a pkg file that I can look at? I have some ideas, but I need to see the model to make sure they will work.
I was going to try hover over the inner face of the shell and clicking tab until all of the appropriate surfaces were highlighted. Then I was going to cut those faces to leave the part as a solid.
I am not sure if this would have worked, but it is worth trying to see.
I made an assumption I shouldn't have. I assumed you shelled with a specified wall thickness, not to a surface part.
I do actually convert from surface parts to solid parts by just filling the empty surface. If you do not mind sharing the shelled part, I would like to look at it.