Try to see if you can find sheet metal 0.4-0.5 mm thick. Cut a piece long enough, and wrap it around the seat post. Maybe that would work?

I've never tried that myself, so I have no idea, but it could be worth a shot.

The dimensions would have to be 83 mm in one direction (that's the circumference of the 26.2 mm seat post, with a tiny margin), and as long as you feel is required in the vertical direction. I'd go for a piece long enough to give support to the seat post all the way down. Maybe 150-250 mm, depending on your bike and configuration.

Oh, and another thing! Make sure you measure both the seat post diameter and seat tube diameter to make sure you have the actual dimensions. The thickness of the sheet metal should be just less than half the difference in diameter.