I'm trying to shell out the inside of a solid feature, but leave behind some structural ribs. The way I'm trying to do this is as follows: for each cut, create two planes--one on the bottom (which can be dimensioned properly), and one parallel to the outside surface (offset by the thickness, 0.1in).

I have a trapezoid on the bottom surface that I'd like to project to the upper plane, then I'd like to