Anything that will hold the bottom of the computer off of the desk surface will help, if you wish to get such a device that includes a fan, feel free to do so.
By the way, I'm sure you've already made all of the decisions about your setup, but for home use, it would be highly advisable to use the PowerBook along with an external monitor, keyboard and mouse. Using the external keyboard and mouse is not only more efficient, but it will make it far less inconvenient to have the computer elevated on some cooling platform.