Potter or Mason Wasps make a mud nest, that this reminds me of. It does have one opening (like a little pot or vase), however I believe they do seal them after stuffing some juicy caterpillars in there for the baby. This could be unfinished or abandoned, or maybe something else entirely. But potter wasp was the first thing I thought of.
http://sites.naturalsciences.org/notebook/inverts/potter_wasp.html
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