Overflow box help/questions

[Milad]

I wanted to get an overflow box going on my tank.
Currently at one end, 10inches down and 5 inches from the walls there are two 1.5in holes drilled with bulkheads

If i built an overlfow box, I would have to put it 8inches deep into the tank and I dont want to take up that much room inside the tank.

What I was thinking is building a overflow box outside the tank and letting the 1.5in holes just carry the water to the overflow box and then having 3 1in returns back to the sump.

What do you guys think? is there an obvious flaw im missing?
Here is a model of the idea.