  Inspired by the first boron-carbon (B-C) clathrate SrB$_3$C$_3$ and the
ternary borohydride KB$_2$H$_8$ [Miao et al., Phys. Rev. B 104 L100504 (2021)],
we have performed first-principles density functional theory calculations of
the electronic and phonon band structures for B-C compounds MB$_2$C$_8$ (M =
Na, K, Rb, Cs). Our calculations reveal that these materials are dynamically
stable and can potentially exhibit superconductivity at ambient pressure.
However, only the K, Rb, and Cs compounds exhibit thermodynamic stability below
50 GPa, while NaB$_2$C$_8$ remains thermodynamically unstable at all pressures
considered. Based on the Allen and Dynes modified McMillan equation, we predict
the superconducting transition temperature $T_c$ of these compounds to be over
65 K at ambient pressure, with $T_c$ decreasing under higher pressures.
Remarkably, we find CsB$_2$C$_8$ possesses the highest predicted $T_c$ of 68.76
K. Our findings demonstrate the possibility of high temperature
superconductivity in cubic MB$_2$C$_8$ at ambient pressure, expanding the B-C
clathrate superconductor family. These results provide valuable insights to
guide the identification of new atmospheric pressure superconductors.
