Note: I updated the scad file to make it a bit easier to maintain. The code is cleaner since I learned the module command. :-) This OpenSCAD file (requires OpenSCAD 2014.03 or newer) can be used to generate a model of box with a sliding lid. You can easily edit constants in the OpenSCAD file to adjust the box's dimensions, wall thickness, and even add a small "button" on the lid to make it easier to open. Adjust the values as necessary, export as STL, and import into your favorite slicer/Repetier Host. Depending on the quality of your printer (or if you print a brim), you may need to trim the lid slightly. No support structure should be needed as the lid angles are all 45º. Open the "lidded box - polyhedron.scad" file into OpenSCAD 2014.03 or newer. Adjust the dimensions, wall thickness, etc. Export as STL and print.