An example mold assembly system includes a mold assembly including a mold forming a bottom of the mold assembly, a funnel operatively coupled to the mold, and an infiltration chamber defined at least partially by the mold and the funnel, the infiltration chamber being used for forming an infiltrated downhole tool. A mold assembly cap is positionable on the mold assembly and including a sidewall extendable about an outer periphery of the mold assembly at least partially along a height of the mold assembly. The sidewall exhibits a horizontal cross-sectional shape that accommodates a shape of the mold assembly and the sidewall is made of a thermal material that promotes directional solidification of the infiltrated downhole tool during fabrication.