Expanding bicycle water bottle threaded insert that installs with a cone wrench. Printed in TPU. It barely fits in the holes in my frame but works well enough. The slot gap works quite well to allow expansion inside the frame. I embedded a M4 washer in the head section, and you can see the M4 * 6mm heatset insert. It feels pretty solid all fastened down. Using Philips head on a proper road bike though... That has got to be fixed. M4 is smaller than typical for bike water bottles, but that is already stretching what is possible for brass inserts and the available wall thickness I had for the surrounding print. Other bikes will have different dimensionality for the hole, frame thickness, and surrounding structure, so sharing the file has no real value. The idea might be useful to someone. It does seem to work and doesn't require the special inserts and tool for installing metal inserts.
Great idea, gentle on the fiber too