Every thing you need (schematics, libraries, pcb) to built a USB to RS232 converter is available on SparkFun web site.
What I found most interesting is a course on Eagle, a well known CAD/CAM software. More than its simplicity, Eagles fame is due to the free availability of a (limited functions) downloadable version.

Yeah it has limited functionalities but how much does a commercial full featured pcb layout software cost?
So, if you have a couple spare hours, you could learn how to use Eagle and in the meanwhile design your own USB-Serial converter using this good free CAD, from schematics to pcb.