An overflow box? Or a skimmer box? An overflow box is used if you have a sump, but your tank is not drilled. A skimmer box is used to pull the surface of the water so the skimmer can pick it up.
I JUST bought one of those things. I am using it on the MJ1200 that powers my HOB refugium. Works good. I have a ton of evaporation daily, so I have to keep up on it at least daily or the skimmer box will run dry.
So far, I like it. I can SEE the oil slick on the surface of the tank flowing into the skimmer box after feeding time. Skimmer activity is WAY up. Filling the cup every 2 days now.
I thought it was kinda expensive, but I knew I needed it. You could build one with plexiglass, a torch and a router.