To address you concern about boron, there is a cubic diamond form of boron nitride $\ce{c-BN}$, ICSD #182731 [1], posseses $V_\mathrm{cell} = \pu{7.99 Å3}$, $Z = 2$ and molar volume
$$V_\mathrm{m} = \frac{N_\mathrm{A}V_\mathrm{cell}}{Z} = \frac{\pu{6.022e23 mol-1}\cdot\pu{7.99 Å3}}{2} \approx \pu{2.406e-6 m3 mol}$$
which is about $30\%$ less than diamond. The only drawback is that this form of boron nitride is predicted to be stable above $\pu{11 Mbar}$.
References
- Qiu, S. L.; Marcus, P. M. Structure and Stability under Pressure of Cubic and Hexagonal Diamond Crystals of C, BN and Si from First Principles. Journal of Physics: Condensed Matter 2011, 23 (21), 215501. https://doi.org/10.1088/0953-8984/23/21/215501.