You didn't post a photo, but I suspect the stem to which you are referring is like this one.

It uses a bolt to press a ridged wedge against the stem. This stem is designed for bars with a 26.4mm diameter at the sleeve. If you're getting that much slippage your bars are probably 25.4mm instead of 26.0mm or 26.4mm.

I would think a shim of something .5mm thick would work, but I don't think an aluminum can is that thick. I suppose it couldn't hurt to try a shim (or doubled-up shim) of can or similar. "Mind you don't cut yourself, Mordecai."