The sides use a living hinge bend and it's all held together with T-Slots, nuts and bolts.
I've attached the files for laser cutting. The original is an Inkscape SVG file. If your laser cutter uses CorelDraw or illustrator, it might be best to open the file in Inkscape and export as DXF. I find DXF files open well in Corel.
The construction is identical to the smaller box I made for the wireless sensor transmitters, which use an ATTiny instead of an Arduino. Follow the link for a lot more detail. The larger box has mounting holes for an Arduino and a hole in the lid for the antenna.