I wanted an enclosure for the UM2 that allowed the electronics to breathe more. I had a hard time getting this to work. At first I tried to put all of the holes in the bottom face in the part model. But all those little perimeters made it difficult to print. So I made the bottom side of the enclosure solid in the model. In the slicer software (S3D), I set the number of top and bottom layers both to zero. And I set the number of shells to 10. It didn't print perfect, but it printed better than any of my previous attempts.