I also have a 46 gallon bowfront with a sump beneath. I use a Mag 5 pump in my sump to return to my tank. My pump discharge goes up about 3 feet, and goes thru 4 elbows.
I tried making my own overflow, the box itself was working fine but the tubing I used didn't. I was using soft tubing that would collapse and/or crack over the course of a month, restricting flow back to my sump which would pump dry. Not a cool situation, over pumped my tank once and filled it with bubbles as my pump cavitated and sucked air a number of times. To compensate I bypassed a lot of my pump discharge back to my sump, and really defeated the whole purpose.
I finally sucked it up and went to my LFS to buy an overflow. It's made by Proclear Aquatic Systems, has a single clear hard-plastic overflow tube. It handles the full flow of my Mag-5 pump perfectly, it maintains a high enough velocity that air bubbles don't get trapped in the top of the U. The only dislike I have for it is the collection reservior in the tank is huge.
It's still a pretty simple project, don't get discouraged.