I use 2 Mag7 pumps to do water changes (1 removes water from the aquarium while the other pumps in new water from a storage tank). I plug them in and walk away during the process so it never hurts to have a fail-safe in the aquarium if the rates vary.
The below is a variation of a few models I have seen on the net. Its made with 3/4" PVC (a bigger size can be used if you need to remove a higer water volume quicker). For the return, I added a PVC-to-garden-hose adapter on the one end and have a garden hose that goes to my laundry room drain.
The two pieces of pipe with the suction cups on them are 8". The piece that rises above the PVC to garden hose adapter is also 8" with the goal of having it higher than the U at the top. The purpose of this piece is to be able to pour water into it to prime the 2nd portion of the outflow. The reason it is higher then the rest is so you can reach it when installed behind an aquarium. Once primed, I added a cap to it but did NOT glue the cap on in case I need to prime in the future.
The biggest bottleneck in the return is the U that hangs over the aquarium. If air gets trapped in there, the siphon stops and its difficult to get the water in that section. To overcome this, I put a hole there and added an aquarium air pump check valve and this works great! I used a 13/64 drill bit and made the hole, I put the check valve in with a small piece of air line tubing attached You should be able to draw air from the valve but not blow air in). I crazy-glued the opening AFTER the valve was in.
When you prime the outflow for the first time, you may have to suck some air out through the check-valve to get it going and in the future if the siphon stops working.
I highly recommend a dry-fit test run. At this point, you can determine the desirable water line for your aquarium based on the length of the PVC pipe in the aquarium (I had to cut the length a few times to get it where I wanted it). If you look close, you can see water coming out of the outtake! (note: tank decor is from my plastic plant days)
After my dry-fit test run, I twisted the joints to the configuration below to best fit the back-left of my aquarium. I then PVC glued the connections (that glue dries in 5 seconds so be quick!). Paint is optional and MUST be waterproof.