Overflow Question

[BeerShark]

Ok, ive looked at a few overflow systems and decided to build the type with the hole drilled at the back with two boxes around it (not sure of the name) one box with no top on it and another smaller one that slots inside with no bottom. what is the purpose of the second smaller box? i gathered it was to stop anything that might float from getting in or possibly to reduce noise?

alos wish me luck on drilling my tank... noone on this whole island will do it