Notebook coolers aren't exactly expensive, but if you're feeling handy, it doesn't take much effort to build your own. Rather than go with something elaborate, you can get by with an elevated platform and a fan to circulate air under the chassis.
If you need a tutorial — no problem. Instructables user TomRay has thrown together a step-by-step guide. The main steps are putting the electronics together and carving up the platform to fit the fan in.
TomRay recommends wood fibre board or plastic and you'll need to hit up eBay or your local electronics store for the other components — a PWM controller, 12V fan and an appropriate power supply.
Check out TomRay's guide for a full explanation.
High power Laptop cooling pad with dust filter [Instructables]