What is it?

This two inch long thing appeared on a steel beam above where I do morning stretching exercises. It's yet another unusual something we've never seen before. I tried to find a similar picture on the internet but failed. A small opening can be seen on the right end, under the decorative arch. It's stationary. My guess is it's some type of nest for insect eggs, or maybe a transformation chamber. I hope I'm lucky enough to see what emerges.