In the International Criticality Safety Benchmark Evaluation Project (ICSBEP) only a limited amount of critical experiments representative of MOX fuel with low moderation is present. This paper aims at describing the design of a set of critical experiments with varying neutron spectra that should contribute to nuclear data validation for this kind of configurations. Complementary to the few existing experiments in the ICSBEP, a series of independent experiments can be performed on the PLANET machine at NCERC (National Criticality Experiments Research Center) using the uranium-plutonium fuel plates. Due to the decay heat emitted by the fuel plates, a dedicated thermal-mechanical design of the assembly has been defined to fit all the experimental constraints: weight, limited temperature and temperature gradient within the assembly, minimal effect on neutronics and others. The 5 critical configurations designs are described in this paper.