loads of bikes have 28.6mm seatposts these days.

a foot and a half...thats rather long

longest ive seen advertised is 400mm
some small folders have longer than that. But they are thicker.

I welded in a seat tube from a old frame into my custom Grifter.
As that was the standard size
Heres my custom grifter.

Have to get round to respraying it and finishing it.

Also heres my Raleigh wildcat. Well it was mine before it got stolen.

Its got a seat tube pushed in then clamped up.
used the seat post that came with the donor bike.