I got the original design from here:
I printed the original design, and I did not like it because it is way too flimsy :)
I heavily modified it for more strength; thicker arms, stronger bottom, and middle plate (because of the walls around it). It houses 2 compartments. The bottom one for the ESCs, and the top compartment for the rest of the electronics, like the flight controller, receiver, etc.
I am using the Naze32 flight controller, Turnigy 9X receiver, and NTM Prop Drive Series 28-26A 1200kv motors.
The wires of the motors go through the interior of the arms, it is a very tight fit! If you don't like it, you are welcome to make more modifications. You will probably have to modify the arms design if you use other motors.
Download the "Quad.skp" and open in SketchUp to see how everything goes together. I also posted the SketchUp files for each individual parts, along with it's STLs.
I left the front of the quadcopter open because I plan to design a case to fit in front of the quad for GoPro camera, and later even a gimbal for the GoPro (not sure if the quad can lift the camera with the gimbal yet)
The battery goes under the quad, between the legs, there is enough space to put a 4000 mAh 4S battery, but make sure whatever battery you use it is less than 118 mm (as that is the space between the legs)
I had a lot of problems printing the big parts, the solution is to slow down the printing BY A LOT on the first layer, after that you can go full speed!
I printed the arms at 75% infill, and the rest at 25% infill.
My printer is the Mendel90 dibond.