Save yourself the headaches and potential disasters and buy a real overflow box. It would be money well spent. I'm all for DIY, but some things are better to buy already made and proven to work. Reef Aquarium Filtration: Eshopps Overflow Boxes
I'll let you know how it works next week. I ordered one and it will be at my house today. I'll be setting up the 75g next week. I was going to drill, but didn't trust myself and possibly breaking the tank so I ordered this overflow.
I'm not sure what makes them special but everyone seems to agree on RC they are the safest,most reliable.I don't know about the their overflow boxes but their sumps,skimmers and fuges are awesome!