Why don't you do the Google Imaging and show us what you mean? There are many images on the resulting page. Do you mean this thing?What's the word for those plastic containers you get at a bakery when you buy slices of cake or cupcakes? You'll see some of them if you search on Google images with the Romanian casoleta.
Interesting. Thank you, PaulQ!The linked picture is of "cakes in a see-through plastic box." - that is an informal term. The formal term is "food-grade plastic container." although I suspect that "food-grade plastic" is better put as some sort of chemical name. This plastic has a greater purity than plastics used for non-food packaging, .i.e there is nothing awful that can leach out into the food.
Clamshell is a pleasantly descriptive term for all containers that open/close in this manner.The trade term for these plastic containers is clamshell.
This very eloquently illustrates that there is no simple English term for such containers. There appears to be no alternative, even within the food packaging industry, to using a cumbersome descriptive sequence.According to the Webstaurantstore.com these are called "pie wedge takeout containers".