I'm actually in a similar predicament and was hopeful the OCZ adapter would be a cheap solution. However, it lacks the bottom 3.5" mounting holes so it may or may not work with your drive rails. This is should definitely work though, albeit at 2-3 times the cost:
Newegg.ca - ICY DOCK MB882SP-1S-2B SSD & SATA Hard Drive Converter - HDD Accessories