Is building your own sump from acrylic sheets (home depot!) an option? You'd be able to control the exact dimensions that way.
Another option is to forget about hiding the sump. I had a 29 gallon tank I wanted to use, but there's no way it would fit under a 55. So, I compromised. It's lower than the DT and off to the side. To avoid it being a total eyesore, I've made the refugium very lively with almost a dozen types of macro and a rock rubble pile in the middle with some beautiful little feather dusters. It's almost as much fun to watch as the DT, crawling with pods
, and definitely soaking up nutrients.