I agree with scrager. This won't work. I gather your thought process stems from an opinion that the end of a PVC pipe sticking in your water would be more asthetically pleasing than a large box with a visible tube stuck in it. I agree!. I haven't implemented this but I was thinking along the following lines:
I drew this up to help explain the issue with your setup. The image on the left is what you need to have when the main pump isn't running. It is vital for this setup that the siphon tube stay full. Any siphon overflow has to have two reservors for the ends of the siphon tube to sit in otherwide the siphon will break.
The siphon can sit like this for days without the tube emptying because there is even pressure on both sides of the tube.
When you start your main pump water will overflow (thus the name) iinto your box (or in this case tube) and the water level on the inside reservoir will raise. This causes pressure on the water in the tube and the water in the siphon tube will flow downhill over the aquarium wall and into the outside reservior. As soon as this reservoir fills at all it drains down to the sump. This maintains the downhill pressure and the water continues to be moved over the aquarium wall to the sump.
When you turn off the main pump the water from the main tank will will flow over the overflow teeth until it drops to just below the teeth. At this point once the inside reservoir and the outside reservoir get to the same level the flow stops. But the siphon remains.
I haven't come up with ideas yet on how to attach these pieces to the tank or to each other. But it very will could be made out of PVC. It would be cheaper than buying a comercial one and could be made to suit your tastes. I personally don't like the white pvc look on the tank. Savko sells PVC pipe and fittings in black. I buy all PVC that is visible in the tank in black color. It is about 2x the cost of white, but it's still pretty cheap.
Good luck.
