In line with what has already been suggested, here's what I do..
Essentially, I find a a firm hose or pipe that is larger than the hose of the ATO and stuff the ATO hose into the larger pipe (red in the diagram) but making sure that the connection isn't air or water tight. The ATO hose simply sits loosly into the larger pipe that goes into the sump. Really the larger pipe just serves as a guide for the water to flow down into the sump while allowing the outlet of the ATO to be way up above the water line of the ATO reservoir and sump. The guide pipe also eliminates splashing of the ATO water since the outlet is so high up.