I put a nice 26.4mm cinelli stem on an old Trek I'm working on, and to get the ~26mm (not sure of exact size) ttt bars to fit tight, I shimmed it with a V8 can...but it's a little too thin. Are beer cans thicker, are any cans slightly thicker? A steel can would be too thick I imagine, since the V8 shim almost but not quite works.

what about another layer of V8 can?

+1
You can add more layers if you need too

I might try that, I was concerned there might be some slippage but it can't be much worse than it is now.

An accurate shimming solution from working on my guns is using a feeler gauge. These little tools are cheap and they have their thicknesses written on them. Find out how much you need and use the one that comes closest or use a combination to get it exact.

again; z-flashing.