Silicon carbide reinforced aluminum alloy matrix composites offer excellent thermo-mechanical properties and are thus attractive for applications limited by thermal stresses. Open porous silicon carbide preforms are fabricated in this work using polymer wax as pore formers. Two different waxes with different particle size were used to fabricate preforms with different pore structures. Wax content was varied to introduce open porosities up to 64 vol%. Structural characterization was carried out using scanning electron microscopy and micro computed tomography, whereas ultrasound phase spectroscopy was used to determine three longitudinal and three shear elastic constants. The amount of porosity increases with the amount of total wax used. Uniaxial pressing prior to isostatic pressing flattens the pores and as a result the preforms behave transverse isotropically with respect to the press direction. For the same total wax content, while the mixture ratio of two waxes has a minor influence on total porosity, the mixture ratio strongly influences the elastic constants. Optimum elastic constants along all directions are obtained with a mixture of two wax types with a higher content of the larger wax particles.