One option: .. Brompton Saddle Adapter Pin, . on top of a plain top seat Post..
it puts a Horizontal tube clamped on top of the seat post.
Faced forward or behind the regular seat post.
Brompton made it to improve fit on the ONE sized bike they Make.
Then you use a simple saddle clamp fitted to the horizontal tube , rotated so the seat goes ontop of it.
the SAP offers a 2" setback ..