If you want a full length overflow to run along the back of your reef, you'll want a coast-to-coast overflow box. It's four-sided with no backpanel: the front, the ends, and a bottom piece. Usually, the box is 5" tall and 4" deep. Length to be determined by you.
For a peninsula tank, you may want a full-width overflow at one end.
Email me for pricing. Pricing starts at $80.00
* Image included showing the recommended Aquarium-Safe silicone available at Home Depot