What solution for shimming issue?

Ok, I have a 30.9 post but a 31.6 seat tube. Does anyone got a proven way to make a secure shim for this type of arrangement – there isn’t anything available through retail (that I can find at least).

Suggestions welcome.

a drinks can will do it, wonderful things in fact i’ve just come in from the garage after shimming my worn shock bushings with a john smiths can ready for tomorrow.