I was thinking about making one of these for my big aquarium also, but I am going to keep things simple and use internal filters. An overflow box would be perfect for sumps though.
Look at the top rated overflow designs and use those as a template. You might be able to use a cheap used HOB style filter and drill holes in the right places, and cover unused stock holes with acrylic and glue.
One function that the higher cost overflows use is a small hole drilled on top of the intake pipe and a small power head line glued SECURELY into place in the top hole. This power head draws water from the pipe so if the suction breaks, the power head can restart the suction, handy for after a power outage. I have never built or used an overflow box, but hopefully this rambling might help out somehow!
Keep us updated on your thoughts and progress!!