I'd like to get clarification. Are you saying that because we use your public domain block, the resulting shape will also have to be public domain?
That is definitely not the definition of public domain. It is not because a red and blue pixel are in public domain, that the combination of both in a certain shape is.
You should (and I am not even sure you could do otherwise legally) accept creations to have their own license, including the possibility that the creation cannot be used by someone else without permission.