Heat diffuser for ceramic cooktop
Use a aluminum plate or disc that your tagine can sit on. The plate needs a minimum thickness of 0.25 inch, any thinner and it may badly warp. The aluminum will heat very evenly, conduct heat better than cast iron, is much softer than glass or ceramic, and is much lighter than brass or cast iron. 

Just don't leave it by itself on the stove, it can be damaged by overheating, similar to overheating other aluminum based cookware.

You can buy 6061 aluminum plates at metal suppliers, and even online at places like amazon.