Parametric FreeCAD design for a rack mount adapter to mount rack to either a seat tube or seat post. Internal edges are chamfered to be carbon fiber safe. I used PETG but ASA should work just as well. Designed for generic Amazon stainless M4 cap screws and nuts, but that's parametric too. Threaded hole is M5 to match standard bike braze-ons.
I printed with 0.15mm layer height, 3 perimeters and 35% fill. Seems to be pretty strong.
The author hasn't provided the model origin yet.