Dmckay

Hello, I am attempting to slice a very simple picture frame, basically a rectangle 96mm x 120.6mm. 3.2mm in thickness, with a step 6mm from the top. As I try to slice it laying flat, Cura almost refuses to see it as a square with 3.2mm sides and a hollow centre. It will not even see the step. If I tilt it on its side then its good, but I really don't want to print it vertically. Any ideas? Thanks for you help in this. Dan