We present a method for the design of 3D constructions from planar pieces that can be cut easily and cheaply with laser cutters and similar Computer Numerical Control (CNC) machines. By cutting tight slits in intersecting pieces, they can be slid into each other forming stable configurations without any gluing or additional connec- tors. These constructions enable quick prototyping and easy exploration of shapes, and are particularly useful for education. We propose a constraint-based optimization method and computational design framework to facilitate such structures.