In this paper, the design of a dielectric multilayer around a clay sample is presented to achieve a uniform electric field distribution over that sample. This structure is located within a multimode microwave-heating oven and is designed by means of genetic algorithms. The permittivity and geometric values for the sample surrounding layers are the selected parameters to be optimized in order to minimize the ratio between the typical deviation and the absolute value of the electric field. The results demonstrate the improvement of the electric field uniformity over the sample using dielectric layers