Is that 18 gallon or 18" tall? What kind of flow are you wanting to run through the return?
Edit: Or is it an 18"x18"x18" cube? Return flow will dictate pipe sizes and overflow box dimensions.
Edit: Also, is it going to be used for a display tank? Probably going to want to minimize the footprint as much as possible.
The overflow boxes that glass holes sells would be a good design that doesn't take up much space. Their 700 gph unit is probably about the size you would need. http://www.glass-holes.com/700-gph-Overflow-Box-Complete-Kit-gh700kit.htm">http://www.glass-holes.com/700-gph-Overflow-Box-Complete-Kit-gh700kit.htm</a>
Would be pretty easy to fab one up, not much material is needed, and since the bottom of your tank might be tempered you will probably want to go with drilling the back panel. I have a similar setup in my 29g at the office. I will post a pic.