I would not be trying this in 3D Chief unless I had seen cabinet guru Mark do it.  But I am unable to reproduce his magic, even after looking carefully at the file he uploaded here with the solution right in it.   I want to extend the left side toward the rear by 2 inches.   I have a tall 86 inch cabinet, faceframe style with 3/4" stiles, and made a solid with a 3/4 x 2 3/4" rectangular profile and 86 inch height, same as cabinet, then saved it as a symbol, a simple fixture.