Bike seatpost bottom adapter (with FreeCAD source)

When using a seatpost with the diameter that corresponds to the frame hole diameter, the load is distributed through all the length of the seatpost. When using a seatpost of a smaller diameter with an adapter, the load is concentrated in the upper area of the frame the adapter contacts with, while the bottom part of the seatpost doesn't contact the frame and suffers little movements to any side around it. You can't see them with your eyes but they may lead to unpleasant whirring noise noise of the frame on bad roads and also to the frame damage like this:

To prevent this, I think it's a good idea to fix the bottom of the seatpost inside the frame to eliminate any possibility of its movements and also to redistribute at least some of the load from the upper adapter to the bottom of the seatpost. This load would be negligible in comparison with the upper adapter, so I think a plastic adapter can be safely used in the bottom.

I made one with PETG and it works fine. It was designed for D27.2 SR Suntour SP12-NCX, so it has a hole in the middle to allow adjustment of the spring. The diameter of the original seatpost for my frame is 30.8 mm. If you need the same adapter for different sizes, I attached a FreeCAD source and a STEP file so that you can modify dimensions for your seatpost and frame.

It holds firmly on my seatpost, no glue required.

ADDED: seatpost-bottom-adapter_rs_3154-Body for 31.6 mm seatpost holes in the frame.