Shape of the metal : sure. Make it thin, like a wire, and long, if necessary as a helix, and then it can match the heat and current conductivity of the semiconductor. A small optimisation tells you how thin. The supporting material, if any, must not itself leak heat.
Other materials: i'm too far outside my knowledge here. Wiki knows it better than I do.
Did you check what commercial components use? Peltier modules are available, the end plates are of alumina, both materials are semiconductor. If you're lucky, you might use one module as is, or maybe reuse the plates and semiconductors, and replace the brazing alloy.