I don't know what's your budget, but this is a pretty good solution for the price:
http://store.apple.com/us/product/H...uctGroup=external&m.tsConnections=thunderbolt
It's not cheap, but 256GB of SSD through Thunderbolt is quite a nice external scratch disk.
Your Mac doesn't have USB 3.0 so otherwise you could get a USB 2.0 external hard drive for really cheap but performance will be considerably worse than the LaCie Thunderbolt SSD I pointed out (155 MB/s vs 30 MB/s).
Another solution would be to use a second internal drive in your optibay. It's more convenient, as long as you don't mind losing your optical drive.