Inspired by @Thrasymack_136015, I wanted a simplified single print solution to allow my wife to have two bottle holding capacity on her pump. She prefers the Ameda pump flanges that came from the hospital, which are directly compatible with Ameda & Dr. Brown bottles (this is photoed, but both sizes look alike).

I included two stl sizes for the two different bottles:

1. Ameda & DrBrown Bottles - Smaller of the two. Ameda is a snug fit. Dr. Brown a little lose, but still secure.
2. Spectra - Larger of the two. Spectra's included bottles are a tight fit.

You could probably use Specra for all three, but since we don't use the Specra at all, I went with the smaller fit.

I also included the source Fusion 360 file, which is built parametric and the model can easily be resized for other bottle sizes. Edit parameter “HolderInsideDiameter”.

Printed on a Prusa i3 mk3, 0.4mm nozzle, 0.3mm layer height, with PETG, without any supports.