Well I have been asked by pm how I did this so I might as well post my method up here for future reference
Like most things there are a load of different ways to do this but my method needed :
Lathe
USE Post (this won't get a mark on it - if your careful ;-))
Masking tape
Thick tie wraps
Steel rule
Sharp knife
Long nose pliers
Some stiff right angle shape (12" length)
Emory (600 ish)
I will try and explain without pics but any questions, just ask and I will do a pic just of that stage.
Mount post in lathe - bottom sticking out.
Slide shim on leaving it to overlap the end of the post a bit.
Add a layer of masking tape around the shim (this is only surface finish protection for the next stage).
Tie wrap shim to post firmly (3 tie wraps).
Face of end of shim.
Only take very small cuts. A deep cut will cause the shim to move. To fast / long will cause the plastic to melt when it's cut.
Once the end is neat (mine was a bit messy direct from use :-() with the lathe still spinning, put a nice finish on the end with the emory.
Cut the tie wraps and remove the shim.
Put it on the post the other way around and repeat the above until the shim is the desired length.
You should now have a plain sleeve of the right length but to long to clamp around / down on the bars.
Fit the shim over the bars, coil it around tightly and mark where the end overlaps.
Slide the shim back on the post and using the steel rule and the sharp knife, score a deep line along the length of the shim in line with the mark from the above.
Remove the shim off the post.
Use the long nose pliers to hold as much of the plastic you want to remove as possible and bend the shim inwards - the excess will snap off cleanly / easily.
Check the fit on the bars.
Cut a length of emory, about 10" long and 2" wide.
Fold it in half so you have a 10x1x1" emory right angle.
Pit this on the right angled bar and secure the ends with masking tape.
Rub the gap of the shim squarely up and down the right angle.
File the gap enough to provide a neat finish and a couple of mm gap when the shim is wrapped tightly around the bar.
If you understand the above and follow each step you will end up with a shim exactly like mine
If you don't have access to a lathe I think it would be quite possible to use the same method but hold the seat post in a vice and chop to length using a hack saw and file / emory the ends to the right length / until square
You could use the end of the post as a gauge of how level you are - even if you catch it with the file it is hidden in the frame
Hope this helps
WD
) I am going to make one 
