DIY: what material to use as DIY shim seat post?

Definitely alloy drink can or other thin sheet metal as it is not compressible and is not affected by water, oil or grease.

Try a rubber sheet cut to size and rolled around the seatpost. It won't slip. Just an idea.

Done this a few times, including on trailer DIY axle and the standard pop alum held up over a couple years there without issue. Also on a prior seatpost issue.

• TO THE GUYS WHO SAY BUY THE RIGHT SIZE SEATPOST there are manufacturers (AND OTHER APPLICATIONS) who don't think standards are for them.
  Example - just bought a folding e-bike with 36mm seat post. It's also literally twice as long. It spans use from 5'2" TO 6'5" (157-196 cm) (FUNNY!) and (it says) is good to support 395 pounds (180 kg). Well - there you go. Now I want a suspender to go with my hard-tail - and the biggest they have is 31.6. Which means folding or layering that can - which I did before.

However I would also be willing to try thin sheet aluminium/aluminum from the hardware store - and some kinds of plastic sheet used for durability (thick heat pressed packaging keeps it's properties under extremes.)

I also discovered a handy source of stainless in just about that thickness (2mm/79 thou) - hidden inside just about any hanging file folder. I went to throw out a bunch and said - hey -- these would be like 40 US cents apiece. And not only that, but I THINK the folder material would be good for most stenciling apps since it's a hard all through and somewhat water resistant. Where else do you find 20x15 sheets of stencil blanks for free?

So yeah - I think you just cut a bunch and drop them in to make a circle - if it fits. Bend, solder, cut, done!