I want to build a sump filter. I was thinking of instead of useing a overfill box can I use a modified hanging filter with a bulk head attatched to the bottom to draw the water out of the tank?

overflow boxes take water from the top inch of the aquarium water, the reason for this is so if the return pump in your sump
was to fail its not going to (A) overfill your sump with large amounts of water from your main tank and (B) if your hanging filter is pump driven
trying to get the top tank to pump water out at the same rate the sump pump is pumping up is very difficult and really unreliable