Even the inner tube solution isn't an ideal solution. Looking at pictures of other people's bikes with an inner tube hack in-place, the indentation on the back of the seat post makes a nice channel for water and dirt to run down into the frame and right past the inner tube. Hence, a good solution needs 2 parts:
- Seal around top of seat tube that has a "fat section" that will fill the gap caused by the channel in the seat mast.
- Cover for adjustment slot.