You need a quill post made by Atax. It has a expanding wedge at the bottom of the post that grips the walls of the seat tube, just like a stem does to a steerer tube.
The Atax seatpost is hard to find (and quite expensive), especially in good usable condition, as the plastic (Delrin) seat rail clamp components break down and crack apart if exposed to UV light for many years. So look carefully at the condition of the plastic parts before buying a used one. If the look bleached out/grey is in color, that is a bad sign. Look for one where the plastic clamp parts look fresher, as replacement parts of the post is now "unobtainium".......