This message is quite old, but I wanted to respond and share my experience in case it helps others. The SparkFun Soap Box enclosure is too small to fit an FIO. I had success if finding a nice, professional looking enclosure from a company called Hammond Manufacturing. Four of their enclosures have the right dimensions for an Arduino FIO. Here are the model numbers:
1551KBK
1553AABK
These are the black versions. There are various other models with the same design and different colours available.