[Zope] Folder-specific metatypes?

I'm working on a product in Python which implements a folder, but restricts the metatypes of the objects which can be added. How, exactly, do I go about doing this? I've tried overriding the filtered_meta_types method, but I'm returning a list, and it seems that I should return a list of dictionaries instead, but I'm not entirely sure how the metatype is converted into a dictionary. I also noticed the _product_meta_types variable, but rgreps of the zope code didn't reveal anything about this variable. Am I on the right track with these? Thanks.