If your piece of metal is sufficiently thick, the eddy currents will not overheat it to the point of failure. If its surface is sufficiently smooth, there will be no arches jumping off the sharp edges. Crumpled foil is usually very much unlike that, and it gets violently destroyed. A steel coffee cup with a smooth top would likely do just fine.

My microwave oven comes with a metal stand that you're supposed to put into it when doing certain kinds of baking. Likely it's engineered to be massive enough, it has rounded edges everywhere, and is supposedly sized so that there are no pieces that would resonate at 2.4 GHz and thus consume a lot of energy and overheat.