This seems like a fairly inexpensive and easy way to get into making your own hollow cast parts. Unfortunately there isn’t a how-to, but between this video, and the “related” post below, you should be able to make your own. If there is enough interest from our readers I could always do a “Build: Your own rotational molder” for the blog?
About 20 dollars worth of Fisher Price Construx from ebay
A drill (DeWalt’s fit nicely, but anything should work)
A few wood screws to hold down the Construx
A piece of scrap wood
More about DIY: Rotational molding
DIY Rotational Molding