Check out these ready-made overflow boxes from BRS - Just silicone it onto the glass and you got an overflow:
http://www.bulkreefsupply.com/catalo.../view/id/1996/
I would also flip that sump around and have the return section where the drain pipes are now. The drains should drain into the larger section, this way you could have a decent sized skimmer and some live rock in there.