Sorry about that...I missed reading the text below the picture about the overflow box (or lack thereof).
OK...if the upper hole is a drain...then you can make the lower hole a return/input. With the low hole being an input, you would have to be very cautious about controlling backflow in a power failure as it would drain your entire tank back into the sump!
While a check valve works, you might just consider blocking that lower hole all together and running the return over the top edge of the tank. It's just safer.
Given you have only 1 hole to work with to feed your sump, I would run some form of the modded Herbie as discussed on this link.
http://www.canreef.com/vbulletin/showthread.php?t=39629
I am concerned that you won't get enough surface removal without some sort of overflow box...or you'll end up with a really noisy drain...
__________________
400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436