[Closed] Seatpost shim 26.2 to 28.6 - anybody seen one?

I'm hoping to use a 1 1/8" ahead stem for the stoker of my tandem, but the seatpost is 26.2mm. Easy I thought, just get a shim - but I can't find any shims with 26.2 internal diameter. Anybody ever seen one or know where to get them?

Failing that, I wonder how feasible it would be to encourage another shim of the right thickness (1.2mm) to fit round the seatpost - for example, could I get away with opening out a 25.0mm ID 27.4mm OD shim (which is available) a bit, or will it just end up oval?

I know I could bodge it by just wrapping with strips of beer can or whatever, but I'd rather get a proper shim if I can - it's quite important that the stoker bars stay put!

Any hints?

Thanks,
Rob