Try this Free 3D Modeler.
http://www.misfitcode.com/misfitmodel3d
Tools, Select faces,
Drag around the whole shell to highlite it.
Primitives, Extrude,
in the pop up, make sure Make Back Faces is selected.
Give the X,Y,Z settings .001
and you'll have back faces.
The settings for X,Y,Z may need scaled down or up a bit.
It's not a bad little Program.