Depends on how difficult or fancy you want to make it. I've thought about building a very simple but effective one out of PVC or ABS piping.
Take a length of pipe that's about 6" in diameter and cut it in half lengthwise so you have a half round of the same length you started with. Cut teeth in one end of it...
Next, you can either silicone or glue it to your tank wall and then glue a piece of acrylic or PVC sheet at the bottom of it to seal the bottom off.
You want to make it large enough so that the teeth you cut give you enough surface skimming. You can even go larger by using larger diameter pipe, such as the thin walled PVC drainage or sewer piping. The length is totally up to you. Length will be determined by how far below the water line your tank is drilled, how tall of a durso stand you want in the overflow and that's about it.