Loose seatpost and other problems

ChrisNW Seatpost
I badly measured my seat tube, and got a size of 1" (25.4). Bought a seatpost, fitted it and found it difficult to get it tight enough.

After some checking and remeasurement, it appears my seat tube is oval by about 1mm, roughly 25.4 front to back and over 26mm side to side.

What should I do?
I've tried shimming the post out with a piece cut from and old paint tin, but it still slips.
I'm thinking to roughen it a bit but haven't tried that yet.

I don't want to splash out on another seatpost, any suggestions?

Might have the same issue, measured my seat tube last night and it's 26mm across and 26.2mm lengthwise (does that make sense?). Maybe it's just me being anal, a 26mm seatpost should do the trick?