ok.... you won't need an overflow, since it's a "closed" loop....
start with a PVC pips going down about 3/4 of the way to the bottom of the tank for your intake, run that down to the pump, on the oputlet of the pump, plumb with PVC to the scwd, from the scwd, run horizontal lengths to the ends of the tank, at the ends of the tank, use 4 90 degree elbows to plumb up and over the top, then back down into the tank, from there, you can do as you wish with creative plumbing inside the tank.....
Here's the cheapo return I have on my 29 gallon.... The outlets are T joints, or 90 degree bends that I heated with a heat gun and squashed with pliers....