Do you mean an actual siphon, and not an overflow? If so, just make one out of PVC pipe, make a half-h shape (imagine the lower half of a capital "H"). Make sure that you install a valve to stop the flow of water and that you cover the inlet. The valve should be near the bottom of the siphon on the outside, and you may need to drill a hole and fix vinyl tubing with a valve above the valve in the PVC so that you can close the PVC valve, open the tubing valve, and suck until you get the water flowing over the top of the tank. You then close the tubing valve and open the PVC valve to create the siphon.