This is what i use
http://www.reefshops.com/hang-on-ove...e-clone-2.html
This overflow is long/wide so snail or anything else will have hard time blocking it. But nevertheless I use a screener on the intake and a second screener on the bulkhead. Also i use a vacuum cleaner hose (wide) to drain the overflow, and last but not least low return flow from sump to the display tank. This slow water flow is for two reasons: Gives skimmer (in the sump)better chance to process the water available at the moment and keeps usage of electricity low (no need for a high flow pump). Inside the display tank i have a high water movement with Tunze pumps. (low voltage also)
The next step in safety is built in duorso stile oveflow with extra overflow pipes for redundency.