The spin splitting of an altermagnet arises from a rotation that relates its two magnetic sublattices. In known cases it is fixed by the crystal structure. Here we show that guest-host frameworks separate the two roles. In the boron-carbon clathrates MB₃C₃ the ordered cage removes the translation between the two guest sites and enforces cubic i-wave altermagnetism, with six symmetry-enforced nodal planes, for any magnetic guest in the compensated state; strain retunes the class to d- or g-wave. We demonstrate the platform with GdB₃C₃: its compensated antiferromagnetic ground state is a narrow-gap semiconductor with spin splitting of 0.19 eV at the conduction-band edge, and its weakly pinned Néel vector switches the helicity-resolved optical response on and off according to the magnetic point group. Thermodynamic stability at high pressure and dynamical stability at ambient pressure indicate that it can be made by the route established for this family.