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.
Giant Yukon, Trek 1000, Schwinn Super Sport Comp, World Sport and Le Tour IV, Haro Cyclocross
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.