Need a machining margin around a sliced 3d object

[baker519]

I am trying to make a box lid that had a 3d model of a Harley Fatboy. I downloaded a model from the Sketchup Warehouse and converted it to a dxf file. I then went into Cut3d and resized it to approximately 7 inches by 4 inches and 3 inches in z. I specified a top and bottom. (I am only going to use the top). I also wanted a rectangular margin around the model so the completed box lid would be 12x8 inches the margin area was approximately 3/8 inch thick. That went as planned. Then I found out my 1/8 inch ball nose only had about 1 inch of reach. So I went back and sliced the model into 3/4 inch pieces. In the process I lost my margin and end up with a sihoulette around the model. Is there any way to get the rectangular margin on a sliced model. The machining margin is locked to sihoulette in the sliced model. Will send pics if I can get it to work.