I think it's the latter one. But I seriously don't know for sure since it's your creation. But I guess since it's a published work you could call it just "(original) cosplay" and just mention in the information that it's actually your own creation since you wrote the source said character appears in.
I was thinking of cosplaying my own main character when my first manga volume comes out and I do not see that as an original cosplay as it is actually based on something other than just a thought.
I think you should go for it!