This is one overflow I made for my tank. Worked well for the few hours I used it for. If you read my disaster thread, you'll know why. Anyhoo, the only change I'm going to make once I start it up again, is that I'll change the durso inside to 1-1/4" since the overflow piping is 1". I was told this will get rid of the toilet action. Otherwise it is extremely quiet. I have just one tiny hole at the top cap, which I can adjust by twisting it.
If I could change anything else, I may of put another hole at the bottom, with another overflow pipe, just going straight up, a little lower than the top edge. This would be, in the case the durso ever got plugged, there would be a second way for the water to drain down to the sump.