Cut3D not seeing the BOX!

[Barefootboy4]

I'm new to Cut 3D and having a small issue I'm sure someone can help with. I'm trying to cut a model airplane wing. There is a upper and lower half. In cad I built the lower half in a BOX 12" x 45" x 3". The wing loft is flush to the top of the box. When I import to Cut 3D the box and wing mold look perfect. The problem is the finish cut preview wants to cut away all of the box right up to the wing loft and not leave the box. What am I doing wrong???
Anyone out there who can help me out? I'm sure it's a simple fix. I've tried using various margins from "0" to 2" but nothing works.
HELP!!!!!!!!!!!

I have attached the DXF file for review

[Teepaco]

Go back to step two in Cut3D and set the "Depth of Model below Surface" hope this helps