Whose nest is this?

All signs are leading me to believe this is a Carolina Wren’s nest, except one: I thought they nested in cavities. Although… my cousins have a Carolina Wren nest on their porch, and the opening (of the cavity and nest) is made strait down like this one. But theirs is located inside a plastic flower pot. Does anyone else have a guess that they’d like to share as to what kind of bird built this nest?