I use a ø31.8 seat tube, and custom machine up an insert that gets press-fit into the top of the seat tube. That part is then seam welded and the top tube covers half of it. It's indestructo.
The problem with that area is that the wall thickness of the tube in that section is too thin, and it usually ovalises during welding meaning that when you stick the seat post in (which differ in size wildly from model to model, BTW) the fit is awful. It usually means you have to reef on the seat clamp to stop the post from slipping down, and because the fit is so bad the post more than likely rocks inside the seat tube as well.
Hey presto - you work harden that section of seat tube which makes it brittle, concentrates the forces at the bottom of that slot, and before you know it you have a cracked frame.
Now, who's for the long explanation?
Suffice to say my frames don't have that problem.
The other solution is to do the same thing, but make kind of a lug on the
outside of the seat tube to beef up the wall thickness a bit. That can look neat in a ye-olde-school kinda way, too.