Base case fits one Parallax PIR sensor and an Arduino UNO R3. This base case slides on, and is a lid to our 9v power Brick. We also have a top cover .STL for this thing over at badbrick.com/catalog The (4) holes on the sides also fit the Lego round shafts in technic kits. Its also designed so that you can attach jumper wires throught the top to connect up something else. If you don't have a 3D printer but you're interested in getting one of these we can print you one at http://www.badbrick.com/catalogIn MakerWare, I modify the "Medium" settings: Quality: Layer Height = 0.2 and I increase the temp to 240C to let the larger prints fuse better. All other defaults are the same. ** Please note to Maker Ware users. I've downgraded our MW back to 2.1.x and the firmware back to 7.1. For some reason our STLs were not printing correctly with the new 2.2 update. The slicer was printing partial faces that are not in the model.