BabyP, our campus has the same type of thing. Instead of building a booth, you might try a table covered with fabrics in your sorority's colors, and if you have big wood letters or a banner, that could go right in front of the table. Have pamphlets or other handouts that tell about your sorority, rush, etc.
I think my SAI chapter is going to participate in the Organization Fair this year, and I think the above is what we'll be doing.
