Air duct clamp. Was sized to model: https://www.printables.com/model/285440-duct-flange by thinkingalaud. Should fit other standard air ducts.

Additional parts:

2x M5 nuts

2x M5-16 or longer screw

I printed in PLA and have been using it for several weeks. I have not noticed any damages, so PLA should hold fine.  Tolerance for the nut is a little tight, so print accordingly.